linux-kernel - Re: [PATCH] sched/cfs: change initial value of runnable

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Date:   Thu, 25 Jun 2020 14:08:02 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel test robot <rong.a.chen@...el.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Phil Auld <pauld@...hat.com>, Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] sched/cfs: change initial value of runnable_avg

On Thu, 25 Jun 2020 at 12:42, Holger Hoffstätte
<holger@...lied-asynchrony.com> wrote:
>
> On 2020-06-25 11:56, Vincent Guittot wrote:
> > On Thu, 25 Jun 2020 at 11:24, Holger Hoffstätte
> > <holger@...lied-asynchrony.com> wrote:
> >>
> >> On 2020-06-24 17:44, Vincent Guittot wrote:
> >>> Some performance regression on reaim benchmark have been raised with
> >>>     commit 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
> >>>
> >>> The problem comes from the init value of runnable_avg which is initialized
> >>> with max value. This can be a problem if the newly forked task is finally
> >>> a short task because the group of CPUs is wrongly set to overloaded and
> >>> tasks are pulled less agressively.
> >>>
> >>> Set initial value of runnable_avg equals to util_avg to reflect that there
> >>> is no waiting time so far.
> >>>
> >>> Fixes: 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
> >>> Reported-by: kernel test robot <rong.a.chen@...el.com>
> >>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >>> ---
> >>>    kernel/sched/fair.c | 2 +-
> >>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>> index 0424a0af5f87..45e467bf42fc 100644
> >>> --- a/kernel/sched/fair.c
> >>> +++ b/kernel/sched/fair.c
> >>> @@ -806,7 +806,7 @@ void post_init_entity_util_avg(struct task_struct *p)
> >>>                }
> >>>        }
> >>>
> >>> -     sa->runnable_avg = cpu_scale;
> >>> +     sa->runnable_avg = sa->util_avg;
> >>>
> >>>        if (p->sched_class != &fair_sched_class) {
> >>>                /*
> >>>
> >>
> >> Something is wrong here. I woke up my machine from suspend-to-RAM this morning
> >> and saw that a completely idle machine had a loadavg of ~7. According to my
> >
> > Just to make sure: Are you speaking about loadavg that is output by
> > /proc/loadavg or load_avg which is the PELT load ?
>
> /proc/loadavg
>
> >> monitoring system this happened to be the loadavg right before I suspended.
> >> I've reverted this, rebooted, created a loadavg >0, suspended and after wake up
> >> loadavg again correctly ranges between 0 and whatever, as expected.
> >
> > I'm not sure to catch why ~7 is bad compared to correctly ranges
> > between 0 and whatever. Isn't ~7 part of the whatever ?
>
> After wakeup the _baseline_ for loadavg seemed to be the last value before suspend,
> not 0. The 7 then was the base loadavg for a _mostly idle machine_ (just reading
> mail etc.), i.e. it never went below said baseline again, no matter the
> _actual_ load.
>
> Here's an image: https://imgur.com/a/kd2stqO
>
> Before 02:00 last night the load was ~7 (compiled something), then all processes
> were terminated and the machine was suspended. After wakeup the machine was mostly
> idle (9am..11am), yet measured loadavg continued with the same value as before.
> I didn't notice this right away since my CPU meter on the desktop didn't show any
> *actual* activity (because there was none). The spike at ~11am is the revert/reboot.

you have reverted only this patch ?

TBH, there is no link between these 2 metrics and I don't see how the
init value of runnable_avg can impact loadavg. As explained, loadavg
is doing a snapshot of nr_running every 5 seconds whereas the impact
of changing this init value will have disappeared in far less than
300ms most of the time.

Let me try to reproduce this on my system


> After that loadavg became normal again, i.e. representative of the actual load,
> even after suspend/resume cycles.
> I suspend/resume every night and the only thing that changed recently was this
> patch, so.. :)
>
> -h