From: Vincent Guittot <vincent.guittot@linaro.org>
To: Chris Mason <clm@fb.com>
Cc: Rik van Riel <riel@surriel.com>,
Peter Zijlstra <peterz@infradead.org>,
Johannes Weiner <hannes@cmpxchg.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()"
Date: Mon, 26 Oct 2020 17:52:41 +0100 [thread overview]
Message-ID: <CAKfTPtCROc_+rPYm8FGHDNJ-U6h1iZ0nm2+xy+tZ+L1q09h28w@mail.gmail.com> (raw)
In-Reply-To: <B23038A4-945F-48E9-8D38-EABE8204F208@fb.com>
On Mon, 26 Oct 2020 at 17:48, Chris Mason <clm@fb.com> wrote:
>
> On 26 Oct 2020, at 12:20, Vincent Guittot wrote:
>
> > Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
> >> On Mon, 26 Oct 2020 16:42:14 +0100
> >> Vincent Guittot <vincent.guittot@linaro.org> wrote:
> >>> On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@surriel.com> wrote:
> >>
> >>>> Could utilization estimates be off, either lagging or
> >>>> simply having a wrong estimate for a task, resulting
> >>>> in no task getting pulled sometimes, while doing a
> >>>> migrate_task imbalance always moves over something?
> >>>
> >>> task and cpu utilization are not always up to fully synced and may
> >>> lag
> >>> a bit which explains that sometimes LB can fail to migrate for a
> >>> small
> >>> diff
> >>
> >> OK, running with this little snippet below, I see latencies
> >> improve back to near where they used to be:
> >>
> >> Latency percentiles (usec) runtime 150 (s)
> >> 50.0th: 13
> >> 75.0th: 31
> >> 90.0th: 69
> >> 95.0th: 90
> >> *99.0th: 761
> >> 99.5th: 2268
> >> 99.9th: 9104
> >> min=1, max=16158
> >>
> >> I suspect the right/cleaner approach might be to use
> >> migrate_task more in !CPU_NOT_IDLE cases?
> >>
> >> Running a task to an idle CPU immediately, instead of refusing
> >> to have the load balancer move it, improves latencies for fairly
> >> obvious reasons.
> >>
> >> I am not entirely clear on why the load balancer should need to
> >> be any more conservative about moving tasks than the wakeup
> >> path is in eg. select_idle_sibling.
> >
> >
> > what you are suggesting is something like:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4978964e75e5..3b6fbf33abc2 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct
> > lb_env *env, struct sd_lb_stats *s
> > * emptying busiest.
> > */
> > if (local->group_type == group_has_spare) {
> > - if (busiest->group_type > group_fully_busy) {
> > + if ((busiest->group_type > group_fully_busy) &&
> > + !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
> > /*
> > * If busiest is overloaded, try to fill spare
> > * capacity. This might end up creating spare
> > capacity
> >
> > which also fixes the problem for me and alignes LB with wakeup path
> > regarding the migration
> > in the LLC
>
> Vincent’s patch on top of 5.10-rc1 looks pretty great:
>
> Latency percentiles (usec) runtime 90 (s) (3320 total samples)
> 50.0th: 161 (1687 samples)
> 75.0th: 200 (817 samples)
> 90.0th: 228 (488 samples)
> 95.0th: 254 (164 samples)
> *99.0th: 314 (131 samples)
> 99.5th: 330 (17 samples)
> 99.9th: 356 (13 samples)
> min=29, max=358
>
> Next we test in prod, which probably won’t have answers until
> tomorrow. Thanks again Vincent!
Great !
I'm going to run more tests on my setup as well to make sure that it
doesn't generate unexpected side effects on other kinds of use cases.
>
> -chris
next prev parent reply other threads:[~2020-10-26 16:52 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-23 23:49 [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" Chris Mason
2020-10-26 8:39 ` Vincent Guittot
2020-10-26 12:45 ` Chris Mason
2020-10-26 14:24 ` Vincent Guittot
2020-10-26 14:38 ` Rik van Riel
2020-10-26 14:56 ` Vincent Guittot
2020-10-26 15:04 ` Rik van Riel
2020-10-26 15:42 ` Vincent Guittot
2020-10-26 15:54 ` Vincent Guittot
2020-10-26 16:04 ` Rik van Riel
2020-10-26 16:20 ` Vincent Guittot
2020-10-26 16:48 ` Chris Mason
2020-10-26 16:52 ` Vincent Guittot [this message]
2020-10-30 2:10 ` Rik van Riel
2020-10-30 9:16 ` Vincent Guittot
2020-10-26 15:05 ` Chris Mason
2020-10-26 15:18 ` Vincent Guittot
2020-10-26 15:28 ` Chris Mason
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