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    SubjectRe: [PATCH v2 1/2] sched/loadavg: Avoid loadavg spikes caused by delayed NO_HZ accounting
    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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