Fixed MTP to work with TWRP

Documentation/timers/00-INDEX

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+00-INDEX
+	- this file
+highres.txt
+	- High resolution timers and dynamic ticks design notes
+hpet.txt
+	- High Precision Event Timer Driver for Linux
+hpet_example.c
+	- sample hpet timer test program
+hrtimers.txt
+	- subsystem for high-resolution kernel timers
+Makefile
+	- Build and link hpet_example
+NO_HZ.txt
+	- Summary of the different methods for the scheduler clock-interrupts management.
+timekeeping.txt
+	- Clock sources, clock events, sched_clock() and delay timer notes
+timers-howto.txt
+	- how to insert delays in the kernel the right (tm) way.
+timer_stats.txt
+	- timer usage statistics

Documentation/timers/Makefile

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+# List of programs to build
+hostprogs-$(CONFIG_X86) := hpet_example
+
+# Tell kbuild to always build the programs
+always := $(hostprogs-y)

Documentation/timers/NO_HZ.txt

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+		NO_HZ: Reducing Scheduling-Clock Ticks
+
+
+This document describes Kconfig options and boot parameters that can
+reduce the number of scheduling-clock interrupts, thereby improving energy
+efficiency and reducing OS jitter.  Reducing OS jitter is important for
+some types of computationally intensive high-performance computing (HPC)
+applications and for real-time applications.
+
+There are three main ways of managing scheduling-clock interrupts
+(also known as "scheduling-clock ticks" or simply "ticks"):
+
+1.	Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or
+	CONFIG_NO_HZ=n for older kernels).  You normally will -not-
+	want to choose this option.
+
+2.	Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or
+	CONFIG_NO_HZ=y for older kernels).  This is the most common
+	approach, and should be the default.
+
+3.	Omit scheduling-clock ticks on CPUs that are either idle or that
+	have only one runnable task (CONFIG_NO_HZ_FULL=y).  Unless you
+	are running realtime applications or certain types of HPC
+	workloads, you will normally -not- want this option.
+
+These three cases are described in the following three sections, followed
+by a third section on RCU-specific considerations, a fourth section
+discussing testing, and a fifth and final section listing known issues.
+
+
+NEVER OMIT SCHEDULING-CLOCK TICKS
+
+Very old versions of Linux from the 1990s and the very early 2000s
+are incapable of omitting scheduling-clock ticks.  It turns out that
+there are some situations where this old-school approach is still the
+right approach, for example, in heavy workloads with lots of tasks
+that use short bursts of CPU, where there are very frequent idle
+periods, but where these idle periods are also quite short (tens or
+hundreds of microseconds).  For these types of workloads, scheduling
+clock interrupts will normally be delivered any way because there
+will frequently be multiple runnable tasks per CPU.  In these cases,
+attempting to turn off the scheduling clock interrupt will have no effect
+other than increasing the overhead of switching to and from idle and
+transitioning between user and kernel execution.
+
+This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
+CONFIG_NO_HZ=n for older kernels).
+
+However, if you are instead running a light workload with long idle
+periods, failing to omit scheduling-clock interrupts will result in
+excessive power consumption.  This is especially bad on battery-powered
+devices, where it results in extremely short battery lifetimes.  If you
+are running light workloads, you should therefore read the following
+section.
+
+In addition, if you are running either a real-time workload or an HPC
+workload with short iterations, the scheduling-clock interrupts can
+degrade your applications performance.  If this describes your workload,
+you should read the following two sections.
+
+
+OMIT SCHEDULING-CLOCK TICKS FOR IDLE CPUs
+
+If a CPU is idle, there is little point in sending it a scheduling-clock
+interrupt.  After all, the primary purpose of a scheduling-clock interrupt
+is to force a busy CPU to shift its attention among multiple duties,
+and an idle CPU has no duties to shift its attention among.
+
+The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
+scheduling-clock interrupts to idle CPUs, which is critically important
+both to battery-powered devices and to highly virtualized mainframes.
+A battery-powered device running a CONFIG_HZ_PERIODIC=y kernel would
+drain its battery very quickly, easily 2-3 times as fast as would the
+same device running a CONFIG_NO_HZ_IDLE=y kernel.  A mainframe running
+1,500 OS instances might find that half of its CPU time was consumed by
+unnecessary scheduling-clock interrupts.  In these situations, there
+is strong motivation to avoid sending scheduling-clock interrupts to
+idle CPUs.  That said, dyntick-idle mode is not free:
+
+1.	It increases the number of instructions executed on the path
+	to and from the idle loop.
+
+2.	On many architectures, dyntick-idle mode also increases the
+	number of expensive clock-reprogramming operations.
+
+Therefore, systems with aggressive real-time response constraints often
+run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
+in order to avoid degrading from-idle transition latencies.
+
+An idle CPU that is not receiving scheduling-clock interrupts is said to
+be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
+tickless".  The remainder of this document will use "dyntick-idle mode".
+
+There is also a boot parameter "nohz=" that can be used to disable
+dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
+By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
+dyntick-idle mode.
+
+
+OMIT SCHEDULING-CLOCK TICKS FOR CPUs WITH ONLY ONE RUNNABLE TASK
+
+If a CPU has only one runnable task, there is little point in sending it
+a scheduling-clock interrupt because there is no other task to switch to.
+Note that omitting scheduling-clock ticks for CPUs with only one runnable
+task implies also omitting them for idle CPUs.
+
+The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
+sending scheduling-clock interrupts to CPUs with a single runnable task,
+and such CPUs are said to be "adaptive-ticks CPUs".  This is important
+for applications with aggressive real-time response constraints because
+it allows them to improve their worst-case response times by the maximum
+duration of a scheduling-clock interrupt.  It is also important for
+computationally intensive short-iteration workloads:  If any CPU is
+delayed during a given iteration, all the other CPUs will be forced to
+wait idle while the delayed CPU finishes.  Thus, the delay is multiplied
+by one less than the number of CPUs.  In these situations, there is
+again strong motivation to avoid sending scheduling-clock interrupts.
+
+By default, no CPU will be an adaptive-ticks CPU.  The "nohz_full="
+boot parameter specifies the adaptive-ticks CPUs.  For example,
+"nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
+CPUs.  Note that you are prohibited from marking all of the CPUs as
+adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
+online to handle timekeeping tasks in order to ensure that system
+calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
+(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
+user processes to observe slight drifts in clock rate.)  Therefore, the
+boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
+"nohz_full=" mask that includes the boot CPU will result in a boot-time
+error message, and the boot CPU will be removed from the mask.  Note that
+this means that your system must have at least two CPUs in order for
+CONFIG_NO_HZ_FULL=y to do anything for you.
+
+Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies
+that all CPUs other than the boot CPU are adaptive-ticks CPUs.  This
+Kconfig parameter will be overridden by the "nohz_full=" boot parameter,
+so that if both the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter and
+the "nohz_full=1" boot parameter is specified, the boot parameter will
+prevail so that only CPU 1 will be an adaptive-ticks CPU.
+
+Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
+This is covered in the "RCU IMPLICATIONS" section below.
+
+Normally, a CPU remains in adaptive-ticks mode as long as possible.
+In particular, transitioning to kernel mode does not automatically change
+the mode.  Instead, the CPU will exit adaptive-ticks mode only if needed,
+for example, if that CPU enqueues an RCU callback.
+
+Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
+not come for free:
+
+1.	CONFIG_NO_HZ_FULL selects CONFIG_NO_HZ_COMMON, so you cannot run
+	adaptive ticks without also running dyntick idle.  This dependency
+	extends down into the implementation, so that all of the costs
+	of CONFIG_NO_HZ_IDLE are also incurred by CONFIG_NO_HZ_FULL.
+
+2.	The user/kernel transitions are slightly more expensive due
+	to the need to inform kernel subsystems (such as RCU) about
+	the change in mode.
+
+3.	POSIX CPU timers on adaptive-tick CPUs may miss their deadlines
+	(perhaps indefinitely) because they currently rely on
+	scheduling-tick interrupts.  This will likely be fixed in
+	one of two ways: (1) Prevent CPUs with POSIX CPU timers from
+	entering adaptive-tick mode, or (2) Use hrtimers or other
+	adaptive-ticks-immune mechanism to cause the POSIX CPU timer to
+	fire properly.
+
+4.	If there are more perf events pending than the hardware can
+	accommodate, they are normally round-robined so as to collect
+	all of them over time.  Adaptive-tick mode may prevent this
+	round-robining from happening.  This will likely be fixed by
+	preventing CPUs with large numbers of perf events pending from
+	entering adaptive-tick mode.
+
+5.	Scheduler statistics for adaptive-tick CPUs may be computed
+	slightly differently than those for non-adaptive-tick CPUs.
+	This might in turn perturb load-balancing of real-time tasks.
+
+6.	The LB_BIAS scheduler feature is disabled by adaptive ticks.
+
+Although improvements are expected over time, adaptive ticks is quite
+useful for many types of real-time and compute-intensive applications.
+However, the drawbacks listed above mean that adaptive ticks should not
+(yet) be enabled by default.
+
+
+RCU IMPLICATIONS
+
+There are situations in which idle CPUs cannot be permitted to
+enter either dyntick-idle mode or adaptive-tick mode, the most
+common being when that CPU has RCU callbacks pending.
+
+The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
+to enter dyntick-idle mode or adaptive-tick mode anyway.  In this case,
+a timer will awaken these CPUs every four jiffies in order to ensure
+that the RCU callbacks are processed in a timely fashion.
+
+Another approach is to offload RCU callback processing to "rcuo" kthreads
+using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
+offload may be selected via several methods:
+
+1.	One of three mutually exclusive Kconfig options specify a
+	build-time default for the CPUs to offload:
+
+	a.	The CONFIG_RCU_NOCB_CPU_NONE=y Kconfig option results in
+		no CPUs being offloaded.
+
+	b.	The CONFIG_RCU_NOCB_CPU_ZERO=y Kconfig option causes
+		CPU 0 to be offloaded.
+
+	c.	The CONFIG_RCU_NOCB_CPU_ALL=y Kconfig option causes all
+		CPUs to be offloaded.  Note that the callbacks will be
+		offloaded to "rcuo" kthreads, and that those kthreads
+		will in fact run on some CPU.  However, this approach
+		gives fine-grained control on exactly which CPUs the
+		callbacks run on, along with their scheduling priority
+		(including the default of SCHED_OTHER), and it further
+		allows this control to be varied dynamically at runtime.
+
+2.	The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
+	list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
+	3, 4, and 5.  The specified CPUs will be offloaded in addition to
+	any CPUs specified as offloaded by CONFIG_RCU_NOCB_CPU_ZERO=y or
+	CONFIG_RCU_NOCB_CPU_ALL=y.  This means that the "rcu_nocbs=" boot
+	parameter has no effect for kernels built with RCU_NOCB_CPU_ALL=y.
+
+The offloaded CPUs will never queue RCU callbacks, and therefore RCU
+never prevents offloaded CPUs from entering either dyntick-idle mode
+or adaptive-tick mode.  That said, note that it is up to userspace to
+pin the "rcuo" kthreads to specific CPUs if desired.  Otherwise, the
+scheduler will decide where to run them, which might or might not be
+where you want them to run.
+
+
+TESTING
+
+So you enable all the OS-jitter features described in this document,
+but do not see any change in your workload's behavior.  Is this because
+your workload isn't affected that much by OS jitter, or is it because
+something else is in the way?  This section helps answer this question
+by providing a simple OS-jitter test suite, which is available on branch
+master of the following git archive:
+
+git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
+
+Clone this archive and follow the instructions in the README file.
+This test procedure will produce a trace that will allow you to evaluate
+whether or not you have succeeded in removing OS jitter from your system.
+If this trace shows that you have removed OS jitter as much as is
+possible, then you can conclude that your workload is not all that
+sensitive to OS jitter.
+
+Note: this test requires that your system have at least two CPUs.
+We do not currently have a good way to remove OS jitter from single-CPU
+systems.
+
+
+KNOWN ISSUES
+
+o	Dyntick-idle slows transitions to and from idle slightly.
+	In practice, this has not been a problem except for the most
+	aggressive real-time workloads, which have the option of disabling
+	dyntick-idle mode, an option that most of them take.  However,
+	some workloads will no doubt want to use adaptive ticks to
+	eliminate scheduling-clock interrupt latencies.  Here are some
+	options for these workloads:
+
+	a.	Use PMQOS from userspace to inform the kernel of your
+		latency requirements (preferred).
+
+	b.	On x86 systems, use the "idle=mwait" boot parameter.
+
+	c.	On x86 systems, use the "intel_idle.max_cstate=" to limit
+	`	the maximum C-state depth.
+
+	d.	On x86 systems, use the "idle=poll" boot parameter.
+		However, please note that use of this parameter can cause
+		your CPU to overheat, which may cause thermal throttling
+		to degrade your latencies -- and that this degradation can
+		be even worse than that of dyntick-idle.  Furthermore,
+		this parameter effectively disables Turbo Mode on Intel
+		CPUs, which can significantly reduce maximum performance.
+
+o	Adaptive-ticks slows user/kernel transitions slightly.
+	This is not expected to be a problem for computationally intensive
+	workloads, which have few such transitions.  Careful benchmarking
+	will be required to determine whether or not other workloads
+	are significantly affected by this effect.
+
+o	Adaptive-ticks does not do anything unless there is only one
+	runnable task for a given CPU, even though there are a number
+	of other situations where the scheduling-clock tick is not
+	needed.  To give but one example, consider a CPU that has one
+	runnable high-priority SCHED_FIFO task and an arbitrary number
+	of low-priority SCHED_OTHER tasks.  In this case, the CPU is
+	required to run the SCHED_FIFO task until it either blocks or
+	some other higher-priority task awakens on (or is assigned to)
+	this CPU, so there is no point in sending a scheduling-clock
+	interrupt to this CPU.	However, the current implementation
+	nevertheless sends scheduling-clock interrupts to CPUs having a
+	single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
+	tasks, even though these interrupts are unnecessary.
+
+	And even when there are multiple runnable tasks on a given CPU,
+	there is little point in interrupting that CPU until the current
+	running task's timeslice expires, which is almost always way
+	longer than the time of the next scheduling-clock interrupt.
+
+	Better handling of these sorts of situations is future work.
+
+o	A reboot is required to reconfigure both adaptive idle and RCU
+	callback offloading.  Runtime reconfiguration could be provided
+	if needed, however, due to the complexity of reconfiguring RCU at
+	runtime, there would need to be an earthshakingly good reason.
+	Especially given that you have the straightforward option of
+	simply offloading RCU callbacks from all CPUs and pinning them
+	where you want them whenever you want them pinned.
+
+o	Additional configuration is required to deal with other sources
+	of OS jitter, including interrupts and system-utility tasks
+	and processes.  This configuration normally involves binding
+	interrupts and tasks to particular CPUs.
+
+o	Some sources of OS jitter can currently be eliminated only by
+	constraining the workload.  For example, the only way to eliminate
+	OS jitter due to global TLB shootdowns is to avoid the unmapping
+	operations (such as kernel module unload operations) that
+	result in these shootdowns.  For another example, page faults
+	and TLB misses can be reduced (and in some cases eliminated) by
+	using huge pages and by constraining the amount of memory used
+	by the application.  Pre-faulting the working set can also be
+	helpful, especially when combined with the mlock() and mlockall()
+	system calls.
+
+o	Unless all CPUs are idle, at least one CPU must keep the
+	scheduling-clock interrupt going in order to support accurate
+	timekeeping.
+
+o	If there might potentially be some adaptive-ticks CPUs, there
+	will be at least one CPU keeping the scheduling-clock interrupt
+	going, even if all CPUs are otherwise idle.
+
+	Better handling of this situation is ongoing work.
+
+o	Some process-handling operations still require the occasional
+	scheduling-clock tick.	These operations include calculating CPU
+	load, maintaining sched average, computing CFS entity vruntime,
+	computing avenrun, and carrying out load balancing.  They are
+	currently accommodated by scheduling-clock tick every second
+	or so.	On-going work will eliminate the need even for these
+	infrequent scheduling-clock ticks.

Documentation/timers/highres.txt

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+High resolution timers and dynamic ticks design notes
+-----------------------------------------------------
+
+Further information can be found in the paper of the OLS 2006 talk "hrtimers
+and beyond". The paper is part of the OLS 2006 Proceedings Volume 1, which can
+be found on the OLS website:
+http://www.linuxsymposium.org/2006/linuxsymposium_procv1.pdf
+
+The slides to this talk are available from:
+http://tglx.de/projects/hrtimers/ols2006-hrtimers.pdf
+
+The slides contain five figures (pages 2, 15, 18, 20, 22), which illustrate the
+changes in the time(r) related Linux subsystems. Figure #1 (p. 2) shows the
+design of the Linux time(r) system before hrtimers and other building blocks
+got merged into mainline.
+
+Note: the paper and the slides are talking about "clock event source", while we
+switched to the name "clock event devices" in meantime.
+
+The design contains the following basic building blocks:
+
+- hrtimer base infrastructure
+- timeofday and clock source management
+- clock event management
+- high resolution timer functionality
+- dynamic ticks
+
+
+hrtimer base infrastructure
+---------------------------
+
+The hrtimer base infrastructure was merged into the 2.6.16 kernel. Details of
+the base implementation are covered in Documentation/timers/hrtimers.txt. See
+also figure #2 (OLS slides p. 15)
+
+The main differences to the timer wheel, which holds the armed timer_list type
+timers are:
+       - time ordered enqueueing into a rb-tree
+       - independent of ticks (the processing is based on nanoseconds)
+
+
+timeofday and clock source management
+-------------------------------------
+
+John Stultz's Generic Time Of Day (GTOD) framework moves a large portion of
+code out of the architecture-specific areas into a generic management
+framework, as illustrated in figure #3 (OLS slides p. 18). The architecture
+specific portion is reduced to the low level hardware details of the clock
+sources, which are registered in the framework and selected on a quality based
+decision. The low level code provides hardware setup and readout routines and
+initializes data structures, which are used by the generic time keeping code to
+convert the clock ticks to nanosecond based time values. All other time keeping
+related functionality is moved into the generic code. The GTOD base patch got
+merged into the 2.6.18 kernel.
+
+Further information about the Generic Time Of Day framework is available in the
+OLS 2005 Proceedings Volume 1:
+http://www.linuxsymposium.org/2005/linuxsymposium_procv1.pdf
+
+The paper "We Are Not Getting Any Younger: A New Approach to Time and
+Timers" was written by J. Stultz, D.V. Hart, & N. Aravamudan.
+
+Figure #3 (OLS slides p.18) illustrates the transformation.
+
+
+clock event management
+----------------------
+
+While clock sources provide read access to the monotonically increasing time
+value, clock event devices are used to schedule the next event
+interrupt(s). The next event is currently defined to be periodic, with its
+period defined at compile time. The setup and selection of the event device
+for various event driven functionalities is hardwired into the architecture
+dependent code. This results in duplicated code across all architectures and
+makes it extremely difficult to change the configuration of the system to use
+event interrupt devices other than those already built into the
+architecture. Another implication of the current design is that it is necessary
+to touch all the architecture-specific implementations in order to provide new
+functionality like high resolution timers or dynamic ticks.
+
+The clock events subsystem tries to address this problem by providing a generic
+solution to manage clock event devices and their usage for the various clock
+event driven kernel functionalities. The goal of the clock event subsystem is
+to minimize the clock event related architecture dependent code to the pure
+hardware related handling and to allow easy addition and utilization of new
+clock event devices. It also minimizes the duplicated code across the
+architectures as it provides generic functionality down to the interrupt
+service handler, which is almost inherently hardware dependent.
+
+Clock event devices are registered either by the architecture dependent boot
+code or at module insertion time. Each clock event device fills a data
+structure with clock-specific property parameters and callback functions. The
+clock event management decides, by using the specified property parameters, the
+set of system functions a clock event device will be used to support. This
+includes the distinction of per-CPU and per-system global event devices.
+
+System-level global event devices are used for the Linux periodic tick. Per-CPU
+event devices are used to provide local CPU functionality such as process
+accounting, profiling, and high resolution timers.
+
+The management layer assigns one or more of the following functions to a clock
+event device:
+      - system global periodic tick (jiffies update)
+      - cpu local update_process_times
+      - cpu local profiling
+      - cpu local next event interrupt (non periodic mode)
+
+The clock event device delegates the selection of those timer interrupt related
+functions completely to the management layer. The clock management layer stores
+a function pointer in the device description structure, which has to be called
+from the hardware level handler. This removes a lot of duplicated code from the
+architecture specific timer interrupt handlers and hands the control over the
+clock event devices and the assignment of timer interrupt related functionality
+to the core code.
+
+The clock event layer API is rather small. Aside from the clock event device
+registration interface it provides functions to schedule the next event
+interrupt, clock event device notification service and support for suspend and
+resume.
+
+The framework adds about 700 lines of code which results in a 2KB increase of
+the kernel binary size. The conversion of i386 removes about 100 lines of
+code. The binary size decrease is in the range of 400 byte. We believe that the
+increase of flexibility and the avoidance of duplicated code across
+architectures justifies the slight increase of the binary size.
+
+The conversion of an architecture has no functional impact, but allows to
+utilize the high resolution and dynamic tick functionalities without any change
+to the clock event device and timer interrupt code. After the conversion the
+enabling of high resolution timers and dynamic ticks is simply provided by
+adding the kernel/time/Kconfig file to the architecture specific Kconfig and
+adding the dynamic tick specific calls to the idle routine (a total of 3 lines
+added to the idle function and the Kconfig file)
+
+Figure #4 (OLS slides p.20) illustrates the transformation.
+
+
+high resolution timer functionality
+-----------------------------------
+
+During system boot it is not possible to use the high resolution timer
+functionality, while making it possible would be difficult and would serve no
+useful function. The initialization of the clock event device framework, the
+clock source framework (GTOD) and hrtimers itself has to be done and
+appropriate clock sources and clock event devices have to be registered before
+the high resolution functionality can work. Up to the point where hrtimers are
+initialized, the system works in the usual low resolution periodic mode. The
+clock source and the clock event device layers provide notification functions
+which inform hrtimers about availability of new hardware. hrtimers validates
+the usability of the registered clock sources and clock event devices before
+switching to high resolution mode. This ensures also that a kernel which is
+configured for high resolution timers can run on a system which lacks the
+necessary hardware support.
+
+The high resolution timer code does not support SMP machines which have only
+global clock event devices. The support of such hardware would involve IPI
+calls when an interrupt happens. The overhead would be much larger than the
+benefit. This is the reason why we currently disable high resolution and
+dynamic ticks on i386 SMP systems which stop the local APIC in C3 power
+state. A workaround is available as an idea, but the problem has not been
+tackled yet.
+
+The time ordered insertion of timers provides all the infrastructure to decide
+whether the event device has to be reprogrammed when a timer is added. The
+decision is made per timer base and synchronized across per-cpu timer bases in
+a support function. The design allows the system to utilize separate per-CPU
+clock event devices for the per-CPU timer bases, but currently only one
+reprogrammable clock event device per-CPU is utilized.
+
+When the timer interrupt happens, the next event interrupt handler is called
+from the clock event distribution code and moves expired timers from the
+red-black tree to a separate double linked list and invokes the softirq
+handler. An additional mode field in the hrtimer structure allows the system to
+execute callback functions directly from the next event interrupt handler. This
+is restricted to code which can safely be executed in the hard interrupt
+context. This applies, for example, to the common case of a wakeup function as
+used by nanosleep. The advantage of executing the handler in the interrupt
+context is the avoidance of up to two context switches - from the interrupted
+context to the softirq and to the task which is woken up by the expired
+timer.
+
+Once a system has switched to high resolution mode, the periodic tick is
+switched off. This disables the per system global periodic clock event device -
+e.g. the PIT on i386 SMP systems.
+
+The periodic tick functionality is provided by an per-cpu hrtimer. The callback
+function is executed in the next event interrupt context and updates jiffies
+and calls update_process_times and profiling. The implementation of the hrtimer
+based periodic tick is designed to be extended with dynamic tick functionality.
+This allows to use a single clock event device to schedule high resolution
+timer and periodic events (jiffies tick, profiling, process accounting) on UP
+systems. This has been proved to work with the PIT on i386 and the Incrementer
+on PPC.
+
+The softirq for running the hrtimer queues and executing the callbacks has been
+separated from the tick bound timer softirq to allow accurate delivery of high
+resolution timer signals which are used by itimer and POSIX interval
+timers. The execution of this softirq can still be delayed by other softirqs,
+but the overall latencies have been significantly improved by this separation.
+
+Figure #5 (OLS slides p.22) illustrates the transformation.
+
+
+dynamic ticks
+-------------
+
+Dynamic ticks are the logical consequence of the hrtimer based periodic tick
+replacement (sched_tick). The functionality of the sched_tick hrtimer is
+extended by three functions:
+
+- hrtimer_stop_sched_tick
+- hrtimer_restart_sched_tick
+- hrtimer_update_jiffies
+
+hrtimer_stop_sched_tick() is called when a CPU goes into idle state. The code
+evaluates the next scheduled timer event (from both hrtimers and the timer
+wheel) and in case that the next event is further away than the next tick it
+reprograms the sched_tick to this future event, to allow longer idle sleeps
+without worthless interruption by the periodic tick. The function is also
+called when an interrupt happens during the idle period, which does not cause a
+reschedule. The call is necessary as the interrupt handler might have armed a
+new timer whose expiry time is before the time which was identified as the
+nearest event in the previous call to hrtimer_stop_sched_tick.
+
+hrtimer_restart_sched_tick() is called when the CPU leaves the idle state before
+it calls schedule(). hrtimer_restart_sched_tick() resumes the periodic tick,
+which is kept active until the next call to hrtimer_stop_sched_tick().
+
+hrtimer_update_jiffies() is called from irq_enter() when an interrupt happens
+in the idle period to make sure that jiffies are up to date and the interrupt
+handler has not to deal with an eventually stale jiffy value.
+
+The dynamic tick feature provides statistical values which are exported to
+userspace via /proc/stats and can be made available for enhanced power
+management control.
+
+The implementation leaves room for further development like full tickless
+systems, where the time slice is controlled by the scheduler, variable
+frequency profiling, and a complete removal of jiffies in the future.
+
+
+Aside the current initial submission of i386 support, the patchset has been
+extended to x86_64 and ARM already. Initial (work in progress) support is also
+available for MIPS and PowerPC.
+
+	  Thomas, Ingo
+
+
+

Documentation/timers/hpet.txt

@@ -0,0 +1,30 @@
+		High Precision Event Timer Driver for Linux
+
+The High Precision Event Timer (HPET) hardware follows a specification
+by Intel and Microsoft which can be found at
+
+	http://www.intel.com/hardwaredesign/hpetspec_1.pdf
+
+Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision")
+and up to 32 comparators.  Normally three or more comparators are provided,
+each of which can generate oneshot interrupts and at least one of which has
+additional hardware to support periodic interrupts.  The comparators are
+also called "timers", which can be misleading since usually timers are
+independent of each other ... these share a counter, complicating resets.
+
+HPET devices can support two interrupt routing modes.  In one mode, the
+comparators are additional interrupt sources with no particular system
+role.  Many x86 BIOS writers don't route HPET interrupts at all, which
+prevents use of that mode.  They support the other "legacy replacement"
+mode where the first two comparators block interrupts from 8254 timers
+and from the RTC.
+
+The driver supports detection of HPET driver allocation and initialization
+of the HPET before the driver module_init routine is called.  This enables
+platform code which uses timer 0 or 1 as the main timer to intercept HPET
+initialization.  An example of this initialization can be found in
+arch/x86/kernel/hpet.c.
+
+The driver provides a userspace API which resembles the API found in the
+RTC driver framework.  An example user space program is provided in
+file:Documentation/timers/hpet_example.c

Documentation/timers/hpet_example.c

@@ -0,0 +1,294 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <string.h>
+#include <memory.h>
+#include <malloc.h>
+#include <time.h>
+#include <ctype.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <signal.h>
+#include <errno.h>
+#include <sys/time.h>
+#include <linux/hpet.h>
+
+
+extern void hpet_open_close(int, const char **);
+extern void hpet_info(int, const char **);
+extern void hpet_poll(int, const char **);
+extern void hpet_fasync(int, const char **);
+extern void hpet_read(int, const char **);
+
+#include <sys/poll.h>
+#include <sys/ioctl.h>
+
+struct hpet_command {
+	char		*command;
+	void		(*func)(int argc, const char ** argv);
+} hpet_command[] = {
+	{
+		"open-close",
+		hpet_open_close
+	},
+	{
+		"info",
+		hpet_info
+	},
+	{
+		"poll",
+		hpet_poll
+	},
+	{
+		"fasync",
+		hpet_fasync
+	},
+};
+
+int
+main(int argc, const char ** argv)
+{
+	int	i;
+
+	argc--;
+	argv++;
+
+	if (!argc) {
+		fprintf(stderr, "-hpet: requires command\n");
+		return -1;
+	}
+
+
+	for (i = 0; i < (sizeof (hpet_command) / sizeof (hpet_command[0])); i++)
+		if (!strcmp(argv[0], hpet_command[i].command)) {
+			argc--;
+			argv++;
+			fprintf(stderr, "-hpet: executing %s\n",
+				hpet_command[i].command);
+			hpet_command[i].func(argc, argv);
+			return 0;
+		}
+
+	fprintf(stderr, "do_hpet: command %s not implemented\n", argv[0]);
+
+	return -1;
+}
+
+void
+hpet_open_close(int argc, const char **argv)
+{
+	int	fd;
+
+	if (argc != 1) {
+		fprintf(stderr, "hpet_open_close: device-name\n");
+		return;
+	}
+
+	fd = open(argv[0], O_RDONLY);
+	if (fd < 0)
+		fprintf(stderr, "hpet_open_close: open failed\n");
+	else
+		close(fd);
+
+	return;
+}
+
+void
+hpet_info(int argc, const char **argv)
+{
+	struct hpet_info	info;
+	int			fd;
+
+	if (argc != 1) {
+		fprintf(stderr, "hpet_info: device-name\n");
+		return;
+	}
+
+	fd = open(argv[0], O_RDONLY);
+	if (fd < 0) {
+		fprintf(stderr, "hpet_info: open of %s failed\n", argv[0]);
+		return;
+	}
+
+	if (ioctl(fd, HPET_INFO, &info) < 0) {
+		fprintf(stderr, "hpet_info: failed to get info\n");
+		goto out;
+	}
+
+	fprintf(stderr, "hpet_info: hi_irqfreq 0x%lx hi_flags 0x%lx ",
+		info.hi_ireqfreq, info.hi_flags);
+	fprintf(stderr, "hi_hpet %d hi_timer %d\n",
+		info.hi_hpet, info.hi_timer);
+
+out:
+	close(fd);
+	return;
+}
+
+void
+hpet_poll(int argc, const char **argv)
+{
+	unsigned long		freq;
+	int			iterations, i, fd;
+	struct pollfd		pfd;
+	struct hpet_info	info;
+	struct timeval		stv, etv;
+	struct timezone		tz;
+	long			usec;
+
+	if (argc != 3) {
+		fprintf(stderr, "hpet_poll: device-name freq iterations\n");
+		return;
+	}
+
+	freq = atoi(argv[1]);
+	iterations = atoi(argv[2]);
+
+	fd = open(argv[0], O_RDONLY);
+
+	if (fd < 0) {
+		fprintf(stderr, "hpet_poll: open of %s failed\n", argv[0]);
+		return;
+	}
+
+	if (ioctl(fd, HPET_IRQFREQ, freq) < 0) {
+		fprintf(stderr, "hpet_poll: HPET_IRQFREQ failed\n");
+		goto out;
+	}
+
+	if (ioctl(fd, HPET_INFO, &info) < 0) {
+		fprintf(stderr, "hpet_poll: failed to get info\n");
+		goto out;
+	}
+
+	fprintf(stderr, "hpet_poll: info.hi_flags 0x%lx\n", info.hi_flags);
+
+	if (info.hi_flags && (ioctl(fd, HPET_EPI, 0) < 0)) {
+		fprintf(stderr, "hpet_poll: HPET_EPI failed\n");
+		goto out;
+	}
+
+	if (ioctl(fd, HPET_IE_ON, 0) < 0) {
+		fprintf(stderr, "hpet_poll, HPET_IE_ON failed\n");
+		goto out;
+	}
+
+	pfd.fd = fd;
+	pfd.events = POLLIN;
+
+	for (i = 0; i < iterations; i++) {
+		pfd.revents = 0;
+		gettimeofday(&stv, &tz);
+		if (poll(&pfd, 1, -1) < 0)
+			fprintf(stderr, "hpet_poll: poll failed\n");
+		else {
+			long 	data;
+
+			gettimeofday(&etv, &tz);
+			usec = stv.tv_sec * 1000000 + stv.tv_usec;
+			usec = (etv.tv_sec * 1000000 + etv.tv_usec) - usec;
+
+			fprintf(stderr,
+				"hpet_poll: expired time = 0x%lx\n", usec);
+
+			fprintf(stderr, "hpet_poll: revents = 0x%x\n",
+				pfd.revents);
+
+			if (read(fd, &data, sizeof(data)) != sizeof(data)) {
+				fprintf(stderr, "hpet_poll: read failed\n");
+			}
+			else
+				fprintf(stderr, "hpet_poll: data 0x%lx\n",
+					data);
+		}
+	}
+
+out:
+	close(fd);
+	return;
+}
+
+static int hpet_sigio_count;
+
+static void
+hpet_sigio(int val)
+{
+	fprintf(stderr, "hpet_sigio: called\n");
+	hpet_sigio_count++;
+}
+
+void
+hpet_fasync(int argc, const char **argv)
+{
+	unsigned long		freq;
+	int			iterations, i, fd, value;
+	sig_t			oldsig;
+	struct hpet_info	info;
+
+	hpet_sigio_count = 0;
+	fd = -1;
+
+	if ((oldsig = signal(SIGIO, hpet_sigio)) == SIG_ERR) {
+		fprintf(stderr, "hpet_fasync: failed to set signal handler\n");
+		return;
+	}
+
+	if (argc != 3) {
+		fprintf(stderr, "hpet_fasync: device-name freq iterations\n");
+		goto out;
+	}
+
+	fd = open(argv[0], O_RDONLY);
+
+	if (fd < 0) {
+		fprintf(stderr, "hpet_fasync: failed to open %s\n", argv[0]);
+		return;
+	}
+
+
+	if ((fcntl(fd, F_SETOWN, getpid()) == 1) ||
+		((value = fcntl(fd, F_GETFL)) == 1) ||
+		(fcntl(fd, F_SETFL, value | O_ASYNC) == 1)) {
+		fprintf(stderr, "hpet_fasync: fcntl failed\n");
+		goto out;
+	}
+
+	freq = atoi(argv[1]);
+	iterations = atoi(argv[2]);
+
+	if (ioctl(fd, HPET_IRQFREQ, freq) < 0) {
+		fprintf(stderr, "hpet_fasync: HPET_IRQFREQ failed\n");
+		goto out;
+	}
+
+	if (ioctl(fd, HPET_INFO, &info) < 0) {
+		fprintf(stderr, "hpet_fasync: failed to get info\n");
+		goto out;
+	}
+
+	fprintf(stderr, "hpet_fasync: info.hi_flags 0x%lx\n", info.hi_flags);
+
+	if (info.hi_flags && (ioctl(fd, HPET_EPI, 0) < 0)) {
+		fprintf(stderr, "hpet_fasync: HPET_EPI failed\n");
+		goto out;
+	}
+
+	if (ioctl(fd, HPET_IE_ON, 0) < 0) {
+		fprintf(stderr, "hpet_fasync, HPET_IE_ON failed\n");
+		goto out;
+	}
+
+	for (i = 0; i < iterations; i++) {
+		(void) pause();
+		fprintf(stderr, "hpet_fasync: count = %d\n", hpet_sigio_count);
+	}
+
+out:
+	signal(SIGIO, oldsig);
+
+	if (fd >= 0)
+		close(fd);
+
+	return;
+}

Documentation/timers/hrtimers.txt

@@ -0,0 +1,178 @@
+
+hrtimers - subsystem for high-resolution kernel timers
+----------------------------------------------------
+
+This patch introduces a new subsystem for high-resolution kernel timers.
+
+One might ask the question: we already have a timer subsystem
+(kernel/timers.c), why do we need two timer subsystems? After a lot of
+back and forth trying to integrate high-resolution and high-precision
+features into the existing timer framework, and after testing various
+such high-resolution timer implementations in practice, we came to the
+conclusion that the timer wheel code is fundamentally not suitable for
+such an approach. We initially didn't believe this ('there must be a way
+to solve this'), and spent a considerable effort trying to integrate
+things into the timer wheel, but we failed. In hindsight, there are
+several reasons why such integration is hard/impossible:
+
+- the forced handling of low-resolution and high-resolution timers in
+  the same way leads to a lot of compromises, macro magic and #ifdef
+  mess. The timers.c code is very "tightly coded" around jiffies and
+  32-bitness assumptions, and has been honed and micro-optimized for a
+  relatively narrow use case (jiffies in a relatively narrow HZ range)
+  for many years - and thus even small extensions to it easily break
+  the wheel concept, leading to even worse compromises. The timer wheel
+  code is very good and tight code, there's zero problems with it in its
+  current usage - but it is simply not suitable to be extended for
+  high-res timers.
+
+- the unpredictable [O(N)] overhead of cascading leads to delays which
+  necessitate a more complex handling of high resolution timers, which
+  in turn decreases robustness. Such a design still led to rather large
+  timing inaccuracies. Cascading is a fundamental property of the timer
+  wheel concept, it cannot be 'designed out' without unevitably
+  degrading other portions of the timers.c code in an unacceptable way.
+
+- the implementation of the current posix-timer subsystem on top of
+  the timer wheel has already introduced a quite complex handling of
+  the required readjusting of absolute CLOCK_REALTIME timers at
+  settimeofday or NTP time - further underlying our experience by
+  example: that the timer wheel data structure is too rigid for high-res
+  timers.
+
+- the timer wheel code is most optimal for use cases which can be
+  identified as "timeouts". Such timeouts are usually set up to cover
+  error conditions in various I/O paths, such as networking and block
+  I/O. The vast majority of those timers never expire and are rarely
+  recascaded because the expected correct event arrives in time so they
+  can be removed from the timer wheel before any further processing of
+  them becomes necessary. Thus the users of these timeouts can accept
+  the granularity and precision tradeoffs of the timer wheel, and
+  largely expect the timer subsystem to have near-zero overhead.
+  Accurate timing for them is not a core purpose - in fact most of the
+  timeout values used are ad-hoc. For them it is at most a necessary
+  evil to guarantee the processing of actual timeout completions
+  (because most of the timeouts are deleted before completion), which
+  should thus be as cheap and unintrusive as possible.
+
+The primary users of precision timers are user-space applications that
+utilize nanosleep, posix-timers and itimer interfaces. Also, in-kernel
+users like drivers and subsystems which require precise timed events
+(e.g. multimedia) can benefit from the availability of a separate
+high-resolution timer subsystem as well.
+
+While this subsystem does not offer high-resolution clock sources just
+yet, the hrtimer subsystem can be easily extended with high-resolution
+clock capabilities, and patches for that exist and are maturing quickly.
+The increasing demand for realtime and multimedia applications along
+with other potential users for precise timers gives another reason to
+separate the "timeout" and "precise timer" subsystems.
+
+Another potential benefit is that such a separation allows even more
+special-purpose optimization of the existing timer wheel for the low
+resolution and low precision use cases - once the precision-sensitive
+APIs are separated from the timer wheel and are migrated over to
+hrtimers. E.g. we could decrease the frequency of the timeout subsystem
+from 250 Hz to 100 HZ (or even smaller).
+
+hrtimer subsystem implementation details
+----------------------------------------
+
+the basic design considerations were:
+
+- simplicity
+
+- data structure not bound to jiffies or any other granularity. All the
+  kernel logic works at 64-bit nanoseconds resolution - no compromises.
+
+- simplification of existing, timing related kernel code
+
+another basic requirement was the immediate enqueueing and ordering of
+timers at activation time. After looking at several possible solutions
+such as radix trees and hashes, we chose the red black tree as the basic
+data structure. Rbtrees are available as a library in the kernel and are
+used in various performance-critical areas of e.g. memory management and
+file systems. The rbtree is solely used for time sorted ordering, while
+a separate list is used to give the expiry code fast access to the
+queued timers, without having to walk the rbtree.
+
+(This separate list is also useful for later when we'll introduce
+high-resolution clocks, where we need separate pending and expired
+queues while keeping the time-order intact.)
+
+Time-ordered enqueueing is not purely for the purposes of
+high-resolution clocks though, it also simplifies the handling of
+absolute timers based on a low-resolution CLOCK_REALTIME. The existing
+implementation needed to keep an extra list of all armed absolute
+CLOCK_REALTIME timers along with complex locking. In case of
+settimeofday and NTP, all the timers (!) had to be dequeued, the
+time-changing code had to fix them up one by one, and all of them had to
+be enqueued again. The time-ordered enqueueing and the storage of the
+expiry time in absolute time units removes all this complex and poorly
+scaling code from the posix-timer implementation - the clock can simply
+be set without having to touch the rbtree. This also makes the handling
+of posix-timers simpler in general.
+
+The locking and per-CPU behavior of hrtimers was mostly taken from the
+existing timer wheel code, as it is mature and well suited. Sharing code
+was not really a win, due to the different data structures. Also, the
+hrtimer functions now have clearer behavior and clearer names - such as
+hrtimer_try_to_cancel() and hrtimer_cancel() [which are roughly
+equivalent to del_timer() and del_timer_sync()] - so there's no direct
+1:1 mapping between them on the algorithmical level, and thus no real
+potential for code sharing either.
+
+Basic data types: every time value, absolute or relative, is in a
+special nanosecond-resolution type: ktime_t. The kernel-internal
+representation of ktime_t values and operations is implemented via
+macros and inline functions, and can be switched between a "hybrid
+union" type and a plain "scalar" 64bit nanoseconds representation (at
+compile time). The hybrid union type optimizes time conversions on 32bit
+CPUs. This build-time-selectable ktime_t storage format was implemented
+to avoid the performance impact of 64-bit multiplications and divisions
+on 32bit CPUs. Such operations are frequently necessary to convert
+between the storage formats provided by kernel and userspace interfaces
+and the internal time format. (See include/linux/ktime.h for further
+details.)
+
+hrtimers - rounding of timer values
+-----------------------------------
+
+the hrtimer code will round timer events to lower-resolution clocks
+because it has to. Otherwise it will do no artificial rounding at all.
+
+one question is, what resolution value should be returned to the user by
+the clock_getres() interface. This will return whatever real resolution
+a given clock has - be it low-res, high-res, or artificially-low-res.
+
+hrtimers - testing and verification
+----------------------------------
+
+We used the high-resolution clock subsystem ontop of hrtimers to verify
+the hrtimer implementation details in praxis, and we also ran the posix
+timer tests in order to ensure specification compliance. We also ran
+tests on low-resolution clocks.
+
+The hrtimer patch converts the following kernel functionality to use
+hrtimers:
+
+ - nanosleep
+ - itimers
+ - posix-timers
+
+The conversion of nanosleep and posix-timers enabled the unification of
+nanosleep and clock_nanosleep.
+
+The code was successfully compiled for the following platforms:
+
+ i386, x86_64, ARM, PPC, PPC64, IA64
+
+The code was run-tested on the following platforms:
+
+ i386(UP/SMP), x86_64(UP/SMP), ARM, PPC
+
+hrtimers were also integrated into the -rt tree, along with a
+hrtimers-based high-resolution clock implementation, so the hrtimers
+code got a healthy amount of testing and use in practice.
+
+	Thomas Gleixner, Ingo Molnar

Documentation/timers/timekeeping.txt

@@ -0,0 +1,179 @@
+Clock sources, Clock events, sched_clock() and delay timers
+-----------------------------------------------------------
+
+This document tries to briefly explain some basic kernel timekeeping
+abstractions. It partly pertains to the drivers usually found in
+drivers/clocksource in the kernel tree, but the code may be spread out
+across the kernel.
+
+If you grep through the kernel source you will find a number of architecture-
+specific implementations of clock sources, clockevents and several likewise
+architecture-specific overrides of the sched_clock() function and some
+delay timers.
+
+To provide timekeeping for your platform, the clock source provides
+the basic timeline, whereas clock events shoot interrupts on certain points
+on this timeline, providing facilities such as high-resolution timers.
+sched_clock() is used for scheduling and timestamping, and delay timers
+provide an accurate delay source using hardware counters.
+
+
+Clock sources
+-------------
+
+The purpose of the clock source is to provide a timeline for the system that
+tells you where you are in time. For example issuing the command 'date' on
+a Linux system will eventually read the clock source to determine exactly
+what time it is.
+
+Typically the clock source is a monotonic, atomic counter which will provide
+n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over.
+It will ideally NEVER stop ticking as long as the system is running. It
+may stop during system suspend.
+
+The clock source shall have as high resolution as possible, and the frequency
+shall be as stable and correct as possible as compared to a real-world wall
+clock. It should not move unpredictably back and forth in time or miss a few
+cycles here and there.
+
+It must be immune to the kind of effects that occur in hardware where e.g.
+the counter register is read in two phases on the bus lowest 16 bits first
+and the higher 16 bits in a second bus cycle with the counter bits
+potentially being updated in between leading to the risk of very strange
+values from the counter.
+
+When the wall-clock accuracy of the clock source isn't satisfactory, there
+are various quirks and layers in the timekeeping code for e.g. synchronizing
+the user-visible time to RTC clocks in the system or against networked time
+servers using NTP, but all they do basically is update an offset against
+the clock source, which provides the fundamental timeline for the system.
+These measures does not affect the clock source per se, they only adapt the
+system to the shortcomings of it.
+
+The clock source struct shall provide means to translate the provided counter
+into a nanosecond value as an unsigned long long (unsigned 64 bit) number.
+Since this operation may be invoked very often, doing this in a strict
+mathematical sense is not desirable: instead the number is taken as close as
+possible to a nanosecond value using only the arithmetic operations
+multiply and shift, so in clocksource_cyc2ns() you find:
+
+  ns ~= (clocksource * mult) >> shift
+
+You will find a number of helper functions in the clock source code intended
+to aid in providing these mult and shift values, such as
+clocksource_khz2mult(), clocksource_hz2mult() that help determine the
+mult factor from a fixed shift, and clocksource_register_hz() and
+clocksource_register_khz() which will help out assigning both shift and mult
+factors using the frequency of the clock source as the only input.
+
+For real simple clock sources accessed from a single I/O memory location
+there is nowadays even clocksource_mmio_init() which will take a memory
+location, bit width, a parameter telling whether the counter in the
+register counts up or down, and the timer clock rate, and then conjure all
+necessary parameters.
+
+Since a 32-bit counter at say 100 MHz will wrap around to zero after some 43
+seconds, the code handling the clock source will have to compensate for this.
+That is the reason why the clock source struct also contains a 'mask'
+member telling how many bits of the source are valid. This way the timekeeping
+code knows when the counter will wrap around and can insert the necessary
+compensation code on both sides of the wrap point so that the system timeline
+remains monotonic.
+
+
+Clock events
+------------
+
+Clock events are the conceptual reverse of clock sources: they take a
+desired time specification value and calculate the values to poke into
+hardware timer registers.
+
+Clock events are orthogonal to clock sources. The same hardware
+and register range may be used for the clock event, but it is essentially
+a different thing. The hardware driving clock events has to be able to
+fire interrupts, so as to trigger events on the system timeline. On an SMP
+system, it is ideal (and customary) to have one such event driving timer per
+CPU core, so that each core can trigger events independently of any other
+core.
+
+You will notice that the clock event device code is based on the same basic
+idea about translating counters to nanoseconds using mult and shift
+arithmetic, and you find the same family of helper functions again for
+assigning these values. The clock event driver does not need a 'mask'
+attribute however: the system will not try to plan events beyond the time
+horizon of the clock event.
+
+
+sched_clock()
+-------------
+
+In addition to the clock sources and clock events there is a special weak
+function in the kernel called sched_clock(). This function shall return the
+number of nanoseconds since the system was started. An architecture may or
+may not provide an implementation of sched_clock() on its own. If a local
+implementation is not provided, the system jiffy counter will be used as
+sched_clock().
+
+As the name suggests, sched_clock() is used for scheduling the system,
+determining the absolute timeslice for a certain process in the CFS scheduler
+for example. It is also used for printk timestamps when you have selected to
+include time information in printk for things like bootcharts.
+
+Compared to clock sources, sched_clock() has to be very fast: it is called
+much more often, especially by the scheduler. If you have to do trade-offs
+between accuracy compared to the clock source, you may sacrifice accuracy
+for speed in sched_clock(). It however requires some of the same basic
+characteristics as the clock source, i.e. it should be monotonic.
+
+The sched_clock() function may wrap only on unsigned long long boundaries,
+i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps
+after circa 585 years. (For most practical systems this means "never".)
+
+If an architecture does not provide its own implementation of this function,
+it will fall back to using jiffies, making its maximum resolution 1/HZ of the
+jiffy frequency for the architecture. This will affect scheduling accuracy
+and will likely show up in system benchmarks.
+
+The clock driving sched_clock() may stop or reset to zero during system
+suspend/sleep. This does not matter to the function it serves of scheduling
+events on the system. However it may result in interesting timestamps in
+printk().
+
+The sched_clock() function should be callable in any context, IRQ- and
+NMI-safe and return a sane value in any context.
+
+Some architectures may have a limited set of time sources and lack a nice
+counter to derive a 64-bit nanosecond value, so for example on the ARM
+architecture, special helper functions have been created to provide a
+sched_clock() nanosecond base from a 16- or 32-bit counter. Sometimes the
+same counter that is also used as clock source is used for this purpose.
+
+On SMP systems, it is crucial for performance that sched_clock() can be called
+independently on each CPU without any synchronization performance hits.
+Some hardware (such as the x86 TSC) will cause the sched_clock() function to
+drift between the CPUs on the system. The kernel can work around this by
+enabling the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK option. This is another aspect
+that makes sched_clock() different from the ordinary clock source.
+
+
+Delay timers (some architectures only)
+--------------------------------------
+
+On systems with variable CPU frequency, the various kernel delay() functions
+will sometimes behave strangely. Basically these delays usually use a hard
+loop to delay a certain number of jiffy fractions using a "lpj" (loops per
+jiffy) value, calibrated on boot.
+
+Let's hope that your system is running on maximum frequency when this value
+is calibrated: as an effect when the frequency is geared down to half the
+full frequency, any delay() will be twice as long. Usually this does not
+hurt, as you're commonly requesting that amount of delay *or more*. But
+basically the semantics are quite unpredictable on such systems.
+
+Enter timer-based delays. Using these, a timer read may be used instead of
+a hard-coded loop for providing the desired delay.
+
+This is done by declaring a struct delay_timer and assigning the appropriate
+function pointers and rate settings for this delay timer.
+
+This is available on some architectures like OpenRISC or ARM.

Documentation/timers/timer_stats.txt

@@ -0,0 +1,73 @@
+timer_stats - timer usage statistics
+------------------------------------
+
+timer_stats is a debugging facility to make the timer (ab)usage in a Linux
+system visible to kernel and userspace developers. If enabled in the config
+but not used it has almost zero runtime overhead, and a relatively small
+data structure overhead. Even if collection is enabled runtime all the
+locking is per-CPU and lookup is hashed.
+
+timer_stats should be used by kernel and userspace developers to verify that
+their code does not make unduly use of timers. This helps to avoid unnecessary
+wakeups, which should be avoided to optimize power consumption.
+
+It can be enabled by CONFIG_TIMER_STATS in the "Kernel hacking" configuration
+section.
+
+timer_stats collects information about the timer events which are fired in a
+Linux system over a sample period:
+
+- the pid of the task(process) which initialized the timer
+- the name of the process which initialized the timer
+- the function where the timer was initialized
+- the callback function which is associated to the timer
+- the number of events (callbacks)
+
+timer_stats adds an entry to /proc: /proc/timer_stats
+
+This entry is used to control the statistics functionality and to read out the
+sampled information.
+
+The timer_stats functionality is inactive on bootup.
+
+To activate a sample period issue:
+# echo 1 >/proc/timer_stats
+
+To stop a sample period issue:
+# echo 0 >/proc/timer_stats
+
+The statistics can be retrieved by:
+# cat /proc/timer_stats
+
+While sampling is enabled, each readout from /proc/timer_stats will see
+newly updated statistics. Once sampling is disabled, the sampled information
+is kept until a new sample period is started. This allows multiple readouts.
+
+Sample output of /proc/timer_stats:
+
+Timerstats sample period: 3.888770 s
+  12,     0 swapper          hrtimer_stop_sched_tick (hrtimer_sched_tick)
+  15,     1 swapper          hcd_submit_urb (rh_timer_func)
+   4,   959 kedac            schedule_timeout (process_timeout)
+   1,     0 swapper          page_writeback_init (wb_timer_fn)
+  28,     0 swapper          hrtimer_stop_sched_tick (hrtimer_sched_tick)
+  22,  2948 IRQ 4            tty_flip_buffer_push (delayed_work_timer_fn)
+   3,  3100 bash             schedule_timeout (process_timeout)
+   1,     1 swapper          queue_delayed_work_on (delayed_work_timer_fn)
+   1,     1 swapper          queue_delayed_work_on (delayed_work_timer_fn)
+   1,     1 swapper          neigh_table_init_no_netlink (neigh_periodic_timer)
+   1,  2292 ip               __netdev_watchdog_up (dev_watchdog)
+   1,    23 events/1         do_cache_clean (delayed_work_timer_fn)
+90 total events, 30.0 events/sec
+
+The first column is the number of events, the second column the pid, the third
+column is the name of the process. The forth column shows the function which
+initialized the timer and in parenthesis the callback function which was
+executed on expiry.
+
+    Thomas, Ingo
+
+Added flag to indicate 'deferrable timer' in /proc/timer_stats. A deferrable
+timer will appear as follows
+  10D,     1 swapper          queue_delayed_work_on (delayed_work_timer_fn)
+

Documentation/timers/timers-howto.txt

@@ -0,0 +1,105 @@
+delays - Information on the various kernel delay / sleep mechanisms
+-------------------------------------------------------------------
+
+This document seeks to answer the common question: "What is the
+RightWay (TM) to insert a delay?"
+
+This question is most often faced by driver writers who have to
+deal with hardware delays and who may not be the most intimately
+familiar with the inner workings of the Linux Kernel.
+
+
+Inserting Delays
+----------------
+
+The first, and most important, question you need to ask is "Is my
+code in an atomic context?"  This should be followed closely by "Does
+it really need to delay in atomic context?" If so...
+
+ATOMIC CONTEXT:
+	You must use the *delay family of functions. These
+	functions use the jiffie estimation of clock speed
+	and will busy wait for enough loop cycles to achieve
+	the desired delay:
+
+	ndelay(unsigned long nsecs)
+	udelay(unsigned long usecs)
+	mdelay(unsigned long msecs)
+
+	udelay is the generally preferred API; ndelay-level
+	precision may not actually exist on many non-PC devices.
+
+	mdelay is macro wrapper around udelay, to account for
+	possible overflow when passing large arguments to udelay.
+	In general, use of mdelay is discouraged and code should
+	be refactored to allow for the use of msleep.
+
+NON-ATOMIC CONTEXT:
+	You should use the *sleep[_range] family of functions.
+	There are a few more options here, while any of them may
+	work correctly, using the "right" sleep function will
+	help the scheduler, power management, and just make your
+	driver better :)
+
+	-- Backed by busy-wait loop:
+		udelay(unsigned long usecs)
+	-- Backed by hrtimers:
+		usleep_range(unsigned long min, unsigned long max)
+	-- Backed by jiffies / legacy_timers
+		msleep(unsigned long msecs)
+		msleep_interruptible(unsigned long msecs)
+
+	Unlike the *delay family, the underlying mechanism
+	driving each of these calls varies, thus there are
+	quirks you should be aware of.
+
+
+	SLEEPING FOR "A FEW" USECS ( < ~10us? ):
+		* Use udelay
+
+		- Why not usleep?
+			On slower systems, (embedded, OR perhaps a speed-
+			stepped PC!) the overhead of setting up the hrtimers
+			for usleep *may* not be worth it. Such an evaluation
+			will obviously depend on your specific situation, but
+			it is something to be aware of.
+
+	SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
+		* Use usleep_range
+
+		- Why not msleep for (1ms - 20ms)?
+			Explained originally here:
+				http://lkml.org/lkml/2007/8/3/250
+			msleep(1~20) may not do what the caller intends, and
+			will often sleep longer (~20 ms actual sleep for any
+			value given in the 1~20ms range). In many cases this
+			is not the desired behavior.
+
+		- Why is there no "usleep" / What is a good range?
+			Since usleep_range is built on top of hrtimers, the
+			wakeup will be very precise (ish), thus a simple
+			usleep function would likely introduce a large number
+			of undesired interrupts.
+
+			With the introduction of a range, the scheduler is
+			free to coalesce your wakeup with any other wakeup
+			that may have happened for other reasons, or at the
+			worst case, fire an interrupt for your upper bound.
+
+			The larger a range you supply, the greater a chance
+			that you will not trigger an interrupt; this should
+			be balanced with what is an acceptable upper bound on
+			delay / performance for your specific code path. Exact
+			tolerances here are very situation specific, thus it
+			is left to the caller to determine a reasonable range.
+
+	SLEEPING FOR LARGER MSECS ( 10ms+ )
+		* Use msleep or possibly msleep_interruptible
+
+		- What's the difference?
+			msleep sets the current task to TASK_UNINTERRUPTIBLE
+			whereas msleep_interruptible sets the current task to
+			TASK_INTERRUPTIBLE before scheduling the sleep. In
+			short, the difference is whether the sleep can be ended
+			early by a signal. In general, just use msleep unless
+			you know you have a need for the interruptible variant.