On 2022/11/9 23:08, Frederic Weisbecker wrote: > On Wed, Nov 09, 2022 at 05:37:38PM +0800, Zhen Lei wrote: >> This commit doucments how to quickly determine the bug causing a given >> RCU CPU stall fault warning based on the output information provided >> by CONFIG_RCU_CPU_STALL_CPUTIME=y. >> >> [ paulmck: Apply wordsmithing. ] >> >> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx> >> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> >> --- >> Documentation/RCU/stallwarn.rst | 88 +++++++++++++++++++++++++++++++++ >> 1 file changed, 88 insertions(+) >> >> diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst >> index dfa4db8c0931eaf..5e24e849290a286 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. > > Is that since the boot or since the last snapshot? Since the last snapshot. See the diagram below: +The sampling period is shown as follows: +|<------------first timeout---------->|<-----second timeout----->| +|<--half timeout-->|<--half timeout-->| | +| |<--first period-->| | +| |<-----------second sampling period---------->| +| | | | +| sampling time point 1st-stall 2nd-stall | | Take the snapshot at this time > >> 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---------->| >> +| | | | >> +| sampling 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. > > Right, unless you're running with CONFIG_NO_HZ_FULL=y and the target CPU > is nohz_full=, in that case you should see a delta in stime because the > cputime is measured with the CPU clock. > > Thanks. > . > -- Regards, Zhen Lei