Fixed MTP to work with TWRP

Documentation/RCU/stallwarn.txt

@@ -0,0 +1,242 @@
+Using RCU's CPU Stall Detector
+
+The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
+detector, which detects conditions that unduly delay RCU grace periods.
+This module parameter enables CPU stall detection by default, but
+may be overridden via boot-time parameter or at runtime via sysfs.
+The stall detector's idea of what constitutes "unduly delayed" is
+controlled by a set of kernel configuration variables and cpp macros:
+
+CONFIG_RCU_CPU_STALL_TIMEOUT
+
+	This kernel configuration parameter defines the period of time
+	that RCU will wait from the beginning of a grace period until it
+	issues an RCU CPU stall warning.  This time period is normally
+	21 seconds.
+
+	This configuration parameter may be changed at runtime via the
+	/sys/module/rcutree/parameters/rcu_cpu_stall_timeout, however
+	this parameter is checked only at the beginning of a cycle.
+	So if you are 10 seconds into a 40-second stall, setting this
+	sysfs parameter to (say) five will shorten the timeout for the
+	-next- stall, or the following warning for the current stall
+	(assuming the stall lasts long enough).  It will not affect the
+	timing of the next warning for the current stall.
+
+	Stall-warning messages may be enabled and disabled completely via
+	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
+
+CONFIG_RCU_CPU_STALL_VERBOSE
+
+	This kernel configuration parameter causes the stall warning to
+	also dump the stacks of any tasks that are blocking the current
+	RCU-preempt grace period.
+
+CONFIG_RCU_CPU_STALL_INFO
+
+	This kernel configuration parameter causes the stall warning to
+	print out additional per-CPU diagnostic information, including
+	information on scheduling-clock ticks and RCU's idle-CPU tracking.
+
+RCU_STALL_DELAY_DELTA
+
+	Although the lockdep facility is extremely useful, it does add
+	some overhead.  Therefore, under CONFIG_PROVE_RCU, the
+	RCU_STALL_DELAY_DELTA macro allows five extra seconds before
+	giving an RCU CPU stall warning message.  (This is a cpp
+	macro, not a kernel configuration parameter.)
+
+RCU_STALL_RAT_DELAY
+
+	The CPU stall detector tries to make the offending CPU print its
+	own warnings, as this often gives better-quality stack traces.
+	However, if the offending CPU does not detect its own stall in
+	the number of jiffies specified by RCU_STALL_RAT_DELAY, then
+	some other CPU will complain.  This delay is normally set to
+	two jiffies.  (This is a cpp macro, not a kernel configuration
+	parameter.)
+
+rcupdate.rcu_task_stall_timeout
+
+	This boot/sysfs parameter controls the RCU-tasks stall warning
+	interval.  A value of zero or less suppresses RCU-tasks stall
+	warnings.  A positive value sets the stall-warning interval
+	in jiffies.  An RCU-tasks stall warning starts wtih the line:
+
+		INFO: rcu_tasks detected stalls on tasks:
+
+	And continues with the output of sched_show_task() for each
+	task stalling the current RCU-tasks grace period.
+
+For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
+it will print a message similar to the following:
+
+INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
+
+This message indicates that CPU 5 detected that it was causing a stall,
+and that the stall was affecting RCU-sched.  This message will normally be
+followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
+RCU and RCU-sched are implemented by the same underlying mechanism,
+while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
+by rcu_preempt_state.
+
+On the other hand, if the offending CPU fails to print out a stall-warning
+message quickly enough, some other CPU will print a message similar to
+the following:
+
+INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
+
+This message indicates that CPU 2 detected that CPUs 3 and 5 were both
+causing stalls, and that the stall was affecting RCU-bh.  This message
+will normally be followed by stack dumps for each CPU.  Please note that
+TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
+and that the tasks will be indicated by PID, for example, "P3421".
+It is even possible for a rcu_preempt_state stall to be caused by both
+CPUs -and- tasks, in which case the offending CPUs and tasks will all
+be called out in the list.
+
+Finally, if the grace period ends just as the stall warning starts
+printing, there will be a spurious stall-warning message:
+
+INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
+
+This is rare, but does happen from time to time in real life.  It is also
+possible for a zero-jiffy stall to be flagged in this case, depending
+on how the stall warning and the grace-period initialization happen to
+interact.  Please note that it is not possible to entirely eliminate this
+sort of false positive without resorting to things like stop_machine(),
+which is overkill for this sort of problem.
+
+If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
+more information is printed with the stall-warning message, for example:
+
+	INFO: rcu_preempt detected stall on CPU
+	0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543
+	   (t=65000 jiffies)
+
+In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
+printed:
+
+	INFO: rcu_preempt detected stall on CPU
+	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
+	   (t=65000 jiffies)
+
+The "(64628 ticks this GP)" indicates that this CPU has taken more
+than 64,000 scheduling-clock interrupts during the current stalled
+grace period.  If the CPU was not yet aware of the current grace
+period (for example, if it was offline), then this part of the message
+indicates how many grace periods behind the CPU is.
+
+The "idle=" portion of the message prints the dyntick-idle state.
+The hex number before the first "/" is the low-order 12 bits of the
+dynticks counter, which will have an even-numbered value if the CPU is
+in dyntick-idle mode and an odd-numbered value otherwise.  The hex
+number between the two "/"s is the value of the nesting, which will
+be a small positive number if in the idle loop and a very large positive
+number (as shown above) otherwise.
+
+The "softirq=" portion of the message tracks the number of RCU softirq
+handlers that the stalled CPU has executed.  The number before the "/"
+is the number that had executed since boot at the time that this CPU
+last noted the beginning of a grace period, which might be the current
+(stalled) grace period, or it might be some earlier grace period (for
+example, if the CPU might have been in dyntick-idle mode for an extended
+time period.  The number after the "/" is the number that have executed
+since boot until the current time.  If this latter number stays constant
+across repeated stall-warning messages, it is possible that RCU's softirq
+handlers are no longer able to execute on this CPU.  This can happen if
+the stalled CPU is spinning with interrupts are disabled, or, in -rt
+kernels, if a high-priority process is starving RCU's softirq handler.
+
+For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
+low-order 16 bits (in hex) of the jiffies counter when this CPU last
+invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
+rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
+prints the number of non-lazy callbacks posted since the last call to
+rcu_needs_cpu().  Finally, an "L" indicates that there are currently
+no non-lazy callbacks ("." is printed otherwise, as shown above) and
+"D" indicates that dyntick-idle processing is enabled ("." is printed
+otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
+
+
+Multiple Warnings From One Stall
+
+If a stall lasts long enough, multiple stall-warning messages will be
+printed for it.  The second and subsequent messages are printed at
+longer intervals, so that the time between (say) the first and second
+message will be about three times the interval between the beginning
+of the stall and the first message.
+
+
+What Causes RCU CPU Stall Warnings?
+
+So your kernel printed an RCU CPU stall warning.  The next question is
+"What caused it?"  The following problems can result in RCU CPU stall
+warnings:
+
+o	A CPU looping in an RCU read-side critical section.
+	
+o	A CPU looping with interrupts disabled.  This condition can
+	result in RCU-sched and RCU-bh stalls.
+
+o	A CPU looping with preemption disabled.  This condition can
+	result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
+	stalls.
+
+o	A CPU looping with bottom halves disabled.  This condition can
+	result in RCU-sched and RCU-bh stalls.
+
+o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
+	kernel without invoking schedule().  Note that cond_resched()
+	does not necessarily prevent RCU CPU stall warnings.  Therefore,
+	if the looping in the kernel is really expected and desirable
+	behavior, you might need to replace some of the cond_resched()
+	calls with calls to cond_resched_rcu_qs().
+
+o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
+	happen to preempt a low-priority task in the middle of an RCU
+	read-side critical section.   This is especially damaging if
+	that low-priority task is not permitted to run on any other CPU,
+	in which case the next RCU grace period can never complete, which
+	will eventually cause the system to run out of memory and hang.
+	While the system is in the process of running itself out of
+	memory, you might see stall-warning messages.
+
+o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
+	is running at a higher priority than the RCU softirq threads.
+	This will prevent RCU callbacks from ever being invoked,
+	and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
+	RCU grace periods from ever completing.  Either way, the
+	system will eventually run out of memory and hang.  In the
+	CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
+	messages.
+
+o	A hardware or software issue shuts off the scheduler-clock
+	interrupt on a CPU that is not in dyntick-idle mode.  This
+	problem really has happened, and seems to be most likely to
+	result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
+
+o	A bug in the RCU implementation.
+
+o	A hardware failure.  This is quite unlikely, but has occurred
+	at least once in real life.  A CPU failed in a running system,
+	becoming unresponsive, but not causing an immediate crash.
+	This resulted in a series of RCU CPU stall warnings, eventually
+	leading the realization that the CPU had failed.
+
+The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
+warning.  Note that SRCU does -not- have CPU stall warnings.  Please note
+that RCU only detects CPU stalls when there is a grace period in progress.
+No grace period, no CPU stall warnings.
+
+To diagnose the cause of the stall, inspect the stack traces.
+The offending function will usually be near the top of the stack.
+If you have a series of stall warnings from a single extended stall,
+comparing the stack traces can often help determine where the stall
+is occurring, which will usually be in the function nearest the top of
+that portion of the stack which remains the same from trace to trace.
+If you can reliably trigger the stall, ftrace can be quite helpful.
+
+RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
+and with RCU's event tracing.  For information on RCU's event tracing,
+see include/trace/events/rcu.h.