Documentation/RCU/stallwarn.rst

7 This document first discusses what sorts of issues RCU's CPU stall
10 this document explains the stall detector's "splat" format.
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
37 	RCU CPU stall warning messages.  Especially if you have added
54 	memory, you might see stall-warning messages.
62 	CONFIG_PREEMPT_RCU case, you might see stall-warning
77 	RCU CPU stall warnings.
79 -	Testing a workload on a fast system, tuning the stall-warning
80 	timeout down to just barely avoid RCU CPU stall warnings, and then
81 	running the same workload with the same stall-warning timeout on a
88 	result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
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.
106 Note that SRCU does -not- have CPU stall warnings.  Please note that
108 No grace period, no CPU stall warnings.
110 To diagnose the cause of the stall, inspect the stack traces.
112 If you have a series of stall warnings from a single extended stall,
113 comparing the stack traces can often help determine where the stall
116 If you can reliably trigger the stall, ftrace can be quite helpful.
127 CPU stall detector, which detects conditions that unduly delay RCU grace
128 periods.  This module parameter enables CPU stall detection by default,
130 The stall detector's idea of what constitutes "unduly delayed" is
138 	issues an RCU CPU stall warning.  This time period is normally
144 	So if you are 10 seconds into a 40-second stall, setting this
146 	-next- stall, or the following warning for the current stall
147 	(assuming the stall lasts long enough).  It will not affect the
148 	timing of the next warning for the current stall.
159 	giving an RCU CPU stall warning message.  (This is a cpp
165 	The CPU stall detector tries to make the offending CPU print its
167 	However, if the offending CPU does not detect its own stall in
176 	This boot/sysfs parameter controls the RCU-tasks stall warning
177 	interval.  A value of zero or less suppresses RCU-tasks stall
178 	warnings.  A positive value sets the stall-warning interval
179 	in seconds.  An RCU-tasks stall warning starts with the line:
199 causing stalls, and that the stall was affecting RCU-sched.  This message
203 possible for an rcu_state stall to be caused by both CPUs -and- tasks,
227 across repeated stall-warning messages, it is possible that RCU's softirq
237 The "detected by" line indicates which CPU detected the stall (in this
254 If the grace period ends just as the stall warning starts printing,
255 there will be a spurious stall-warning message, which will include
261 possible for a zero-jiffy stall to be flagged in this case, depending
262 on how the stall warning and the grace-period initialization happen to
268 grace period has nevertheless failed to end, the stall-warning splat
281 the stall warning, as was the case in the "All QSes seen" line above,
287 in RCU CPU stall warnings even when all CPUs and tasks have passed
299 If a stall lasts long enough, multiple stall-warning messages will be
303 of the stall and the first message.
309 If an expedited grace period detects a stall, it will place a message
335 It is entirely possible to see stall warnings from normal and from