2017-02-17 20:07 GMT+08:00 Matt Fleming <m...@codeblueprint.co.uk>:
> 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 <pet...@infradead.org>
> Cc: Mike Galbraith <umgwanakikb...@gmail.com>
> Cc: Morten Rasmussen <morten.rasmus...@arm.com>
> Cc: Vincent Guittot <vincent.guit...@linaro.org>
> Cc: Frederic Weisbecker <fweis...@gmail.com>
> Cc: <sta...@vger.kernel.org> # v3.5+
> Signed-off-by: Matt Fleming <m...@codeblueprint.co.uk>
Reviewed-by: Wanpeng Li <wanpeng...@hotmail.com>
> ---
> kernel/sched/loadavg.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> Changes in v2:
>
> - Folded in Peter's suggestion for how to fix this.
>
> - Tried to clairfy the changelog based on feedback from Peter and
> Frederic
>
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index a2d6eb71f06b..ec91fcc09bfe 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -201,8 +201,9 @@ void calc_load_exit_idle(void)
> struct rq *this_rq = this_rq();
>
> /*
> - * If we're still before the sample window, we're done.
> + * If we're still before the pending sample window, we're done.
> */
> + this_rq->calc_load_update = calc_load_update;
> if (time_before(jiffies, this_rq->calc_load_update))
> return;
>
> @@ -211,7 +212,6 @@ void calc_load_exit_idle(void)
> * accounted through the nohz accounting, so skip the entire deal and
> * sync up for the next window.
> */
> - this_rq->calc_load_update = calc_load_update;
> if (time_before(jiffies, this_rq->calc_load_update + 10))
> this_rq->calc_load_update += LOAD_FREQ;
> }
> --
> 2.10.0
>
