Messages in this thread | | | Subject | Re: RCU stall when using function_graph | From | Daniel Lezcano <> | Date | Fri, 11 Aug 2017 11:38:09 +0200 |
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On 10/08/2017 23:39, Paul E. McKenney wrote: > On Thu, Aug 10, 2017 at 11:45:09AM +0200, Daniel Lezcano wrote:
[ ... ]
>> Nothing coming in mind but may be worth to mention the slowness of the >> CPU is the aggravating factor. In particular I was able to reproduce the >> issue by setting to the min CPU frequency. With the ondemand governor, >> we can have the frequency high (hence enough CPU power) at the moment we >> set the function_graph because another CPU is loaded (and both CPUs are >> sharing the same clock line). The system became stuck at the moment the >> other CPU went idle with the lowest frequency. That introduced >> randomness in the issue and made hard to figure out why the RCU stall >> was happening. > > Adding this, then?
Yes, sure.
Thanks Paul.
-- Daniel
> ------------------------------------------------------------------------ > > commit f7d9ce95064f76be583c775fac32076fa59f1617 > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Date: Thu Aug 10 14:33:17 2017 -0700 > > documentation: Slow systems can stall RCU grace periods > > If a fast system has a worst-case grace-period duration of (say) ten > seconds, then running the same workload on a system ten times as slow > will get you an RCU CPU stall warning given default stall-warning > timeout settings. This commit therefore adds this possibility to > stallwarn.txt. > > Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt > index 21b8913acbdf..238acbd94917 100644 > --- a/Documentation/RCU/stallwarn.txt > +++ b/Documentation/RCU/stallwarn.txt > @@ -70,6 +70,12 @@ o A periodic interrupt whose handler takes longer than the time > considerably longer than normal, which can in turn result in > RCU CPU stall warnings. > > +o Testing a workload on a fast system, tuning the stall-warning > + timeout down to just barely avoid RCU CPU stall warnings, and then > + running the same workload with the same stall-warning timeout on a > + slow system. Note that thermal throttling and on-demand governors > + can cause a single system to be sometimes fast and sometimes slow! > + > 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 >
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