Documentation/RCU/stallwarn.rst

4 Using RCU's CPU Stall Detector
7 This document first discusses what sorts of issues RCU's CPU stall
13 What Causes RCU CPU Stall Warnings?
16 So your kernel printed an RCU CPU stall warning.  The next question is
17 "What caused it?"  The following problems can result in RCU CPU stall
20 -	A CPU looping in an RCU read-side critical section.
37 	RCU CPU stall warning messages.  Especially if you have added
40 -	Anything that prevents RCU's grace-period kthreads from running.
48 	happen to preempt a low-priority task in the middle of an RCU
51 	in which case the next RCU grace period can never complete, which
57 	is running at a higher priority than the RCU softirq threads.
58 	This will prevent RCU callbacks from ever being invoked,
60 	RCU grace periods from ever completing.  Either way, the
66 	increase the scheduling priority of RCU's kthreads, which can
73 	prevent RCU's kthreads and softirq handlers from running.
77 	RCU CPU stall warnings.
80 	timeout down to just barely avoid RCU CPU stall warnings, and then
88 	result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
95 	yes, even including bugs in RCU itself.
97 -	A bug in the RCU implementation.
102 	This resulted in a series of RCU CPU stall warnings, eventually
105 The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning.
107 RCU only detects CPU stalls when there is a grace period in progress.
118 RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
119 and with RCU's event tracing.  For information on RCU's event tracing,
120 see include/trace/events/rcu.h.
123 Fine-Tuning the RCU CPU Stall Detector
126 The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
127 CPU stall detector, which detects conditions that unduly delay RCU grace
137 	that RCU will wait from the beginning of a grace period until it
138 	issues an RCU CPU stall warning.  This time period is normally
159 	giving an RCU CPU stall warning message.  (This is a cpp
176 	This boot/sysfs parameter controls the RCU-tasks stall warning
177 	interval.  A value of zero or less suppresses RCU-tasks stall
179 	in seconds.  An RCU-tasks stall warning starts with the line:
184 	task stalling the current RCU-tasks grace period.
187 Interpreting RCU's CPU Stall-Detector "Splats"
190 For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
199 causing stalls, and that the stall was affecting RCU-sched.  This message
207 the RCU core for the past three grace periods.  In contrast, CPU 16's "(0
219 The "softirq=" portion of the message tracks the number of RCU softirq
227 across repeated stall-warning messages, it is possible that RCU's softirq
230 kernels, if a high-priority process is starving RCU's softirq handler.
240 an estimate of the total number of RCU callbacks queued across all CPUs
287 in RCU CPU stall warnings even when all CPUs and tasks have passed