Documentation / RCU / stallwarn.txt

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Based on kernel version 3.9. Page generated on 2013-05-02 23:13 EST.

	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
		sixty 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 30 seconds into a 70-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/rcutree/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.
	
	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.
	
	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.
	
	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.
	
	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
		   (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 drain=0 . timer not pending
		   (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.
	
	For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the CPU is
	not in the process of trying to force itself into dyntick-idle state, the
	"." indicates that the CPU has not given up forcing RCU into dyntick-idle
	mode (it would be "H" otherwise), and the "timer not pending" indicates
	that the CPU has not recently forced RCU into dyntick-idle mode (it
	would otherwise indicate the number of microseconds remaining in this
	forced state).
	
	
	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().
	
	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=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, and RCU-bh implementations have CPU stall warning.
	SRCU does not have its own CPU stall warnings, but its calls to
	synchronize_sched() will result in RCU-sched detecting RCU-sched-related
	CPU stalls.  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.

• [ RCU ]
• 00-INDEX
• arrayRCU.txt
• checklist.txt
• listRCU.txt
• lockdep-splat.txt
• lockdep.txt
• NMI-RCU.txt
• rcu.txt
• rcubarrier.txt
• rculist_nulls.txt
• rcuref.txt
• RTFP.txt
• stallwarn.txt
• torture.txt
• trace.txt
• UP.txt
• whatisRCU.txt
•