Messages in this thread | | | Date | Wed, 22 Feb 2017 16:18:05 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v2 1/2] sched/loadavg: Avoid loadavg spikes caused by delayed NO_HZ accounting |
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On Fri, Feb 17, 2017 at 12:07:30PM +0000, Matt Fleming wrote: > If we crossed a sample window while in NO_HZ we will add LOAD_FREQ to > the pending sample window time on exit, setting the next update not > one window into the future, but two. > > This situation on exiting NO_HZ is described by: > > this_rq->calc_load_update < jiffies < calc_load_update > > In this scenario, what we should be doing is: > > this_rq->calc_load_update = calc_load_update [ next window ] > > But what we actually do is: > > this_rq->calc_load_update = calc_load_update + LOAD_FREQ [ next+1 window ] > > This has the effect of delaying load average updates for potentially > up to ~9seconds. > > This can result in huge spikes in the load average values due to > per-cpu uninterruptible task counts being out of sync when accumulated > across all CPUs. > > It's safe to update the per-cpu active count if we wake between sample > windows because any load that we left in 'calc_load_idle' will have > been zero'd when the idle load was folded in calc_global_load(). > > This issue is easy to reproduce before, > > commit 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking") > > just by forking short-lived process pipelines built from ps(1) and > grep(1) in a loop. I'm unable to reproduce the spikes after that > commit, but the bug still seems to be present from code review. > > Fixes: commit 5167e8d ("sched/nohz: Rewrite and fix load-avg computation -- again") > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Mike Galbraith <umgwanakikbuti@gmail.com> > Cc: Morten Rasmussen <morten.rasmussen@arm.com> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Thanks it's much clearer now!
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