From 7a21ddf01af9da0a2d24d6ece0e2ffd60374a945 Mon Sep 17 00:00:00 2001 From: Zhen Lei Date: Sat, 19 Nov 2022 17:25:07 +0800 Subject: [PATCH] doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall information This commit documents the additional RCU CPU stall warning output produced by kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with rcupdate.rcu_cpu_stall_cputime=1. [ paulmck: Apply wordsmithing. ] Signed-off-by: Zhen Lei Reviewed-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney --- Documentation/RCU/stallwarn.rst | 88 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 88 insertions(+) diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst index dfa4db8..c1e92df 100644 --- a/Documentation/RCU/stallwarn.rst +++ b/Documentation/RCU/stallwarn.rst @@ -390,3 +390,91 @@ for example, "P3421". It is entirely possible to see stall warnings from normal and from expedited grace periods at about the same time during the same run. + +RCU_CPU_STALL_CPUTIME +===================== + +In kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with +rcupdate.rcu_cpu_stall_cputime=1, the following additional information +is supplied with each RCU CPU stall warning:: + +rcu: hardirqs softirqs csw/system +rcu: number: 624 45 0 +rcu: cputime: 69 1 2425 ==> 2500(ms) + +These statistics are collected during the sampling period. The values +in row "number:" are the number of hard interrupts, number of soft +interrupts, and number of context switches on the stalled CPU. The +first three values in row "cputime:" indicate the CPU time in +milliseconds consumed by hard interrupts, soft interrupts, and tasks +on the stalled CPU. The last number is the measurement interval, again +in milliseconds. Because user-mode tasks normally do not cause RCU CPU +stalls, these tasks are typically kernel tasks, which is why only the +system CPU time are considered. + +The sampling period is shown as follows: +:<------------first timeout---------->:<-----second timeout----->: +:<--half timeout-->:<--half timeout-->: : +: :<--first period-->: : +: :<-----------second sampling period---------->: +: : : : +: snapshot time point 1st-stall 2nd-stall + + +The following describes four typical scenarios: + +1. A CPU looping with interrupts disabled.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 0 0 0 + rcu: cputime: 0 0 0 ==> 2500(ms) + + Because interrupts have been disabled throughout the measurement + interval, there are no interrupts and no context switches. + Furthermore, because CPU time consumption was measured using interrupt + handlers, the system CPU consumption is misleadingly measured as zero. + This scenario will normally also have "(0 ticks this GP)" printed on + this CPU's summary line. + +2. A CPU looping with bottom halves disabled. + + This is similar to the previous example, but with non-zero number of + and CPU time consumed by hard interrupts, along with non-zero CPU + time consumed by in-kernel execution.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 0 0 + rcu: cputime: 49 0 2446 ==> 2500(ms) + + The fact that there are zero softirqs gives a hint that these were + disabled, perhaps via local_bh_disable(). It is of course possible + that there were no softirqs, perhaps because all events that would + result in softirq execution are confined to other CPUs. In this case, + the diagnosis should continue as shown in the next example. + +3. A CPU looping with preemption disabled. + + Here, only the number of context switches is zero.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 45 0 + rcu: cputime: 69 1 2425 ==> 2500(ms) + + This situation hints that the stalled CPU was looping with preemption + disabled. + +4. No looping, but massive hard and soft interrupts.:: + + rcu: hardirqs softirqs csw/system + rcu: number: xx xx 0 + rcu: cputime: xx xx 0 ==> 2500(ms) + + Here, the number and CPU time of hard interrupts are all non-zero, + but the number of context switches and the in-kernel CPU time consumed + are zero. The number and cputime of soft interrupts will usually be + non-zero, but could be zero, for example, if the CPU was spinning + within a single hard interrupt handler. + + If this type of RCU CPU stall warning can be reproduced, you can + narrow it down by looking at /proc/interrupts or by writing code to + trace each interrupt, for example, by referring to show_interrupts(). -- 2.7.4