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    Subject[PATCH 4.4 088/101] sched/loadavg: Avoid loadavg spikes caused by delayed NO_HZ accounting
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Matt Fleming <matt@codeblueprint.co.uk>

    commit 6e5f32f7a43f45ee55c401c0b9585eb01f9629a8 upstream.

    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.

    Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
    Cc: Morten Rasmussen <morten.rasmussen@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vincent Guittot <vincent.guittot@linaro.org>
    Fixes: commit 5167e8d ("sched/nohz: Rewrite and fix load-avg computation -- again")
    Link: http://lkml.kernel.org/r/20170217120731.11868-2-matt@codeblueprint.co.uk
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/sched/loadavg.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- 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;
    }

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