From: Qais Yousef <qais.yousef@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Paul McKenney <paulmck@kernel.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Dietmar Eggeman <dietmar.eggemann@arm.com>,
Ben Segall <bsegall@google.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ
Date: Fri, 22 Jan 2021 18:39:27 +0000 [thread overview]
Message-ID: <20210122183927.ivqyapttzd6lflwk@e107158-lin> (raw)
In-Reply-To: <CAKfTPtAnzhDKXayicDdymWpK1UswfkTaO8vL-WHxVaoj7DaCFw@mail.gmail.com>
On 01/22/21 17:56, Vincent Guittot wrote:
> > ---
> > 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 04a3ce20da67..fe2dc0024db5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8381,7 +8381,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
> > if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
> > return false;
> >
> > - if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick))
> > + if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick + (HZ/20)))
>
> This condition is there to make sure to update blocked load at most
> once a tick in order to filter newly idle case otherwise the rate
> limit is already done by load balance interval
> This hard coded (HZ/20) looks really like an ugly hack
This was meant as an RFC patch to discuss the problem really.
Joel is seeing update_blocked_averages() taking ~100us. Half of it seems in
processing __update_blocked_fair() and the other half in sugov_update_shared().
So roughly 50us each. Note that each function is calling an iterator in
return. Correct me if my numbers are wrong Joel.
Running on a little core on low frequency these numbers don't look too odd.
So I'm not seeing how we can speed these functions up.
But since update_sg_lb_stats() will end up with multiple calls to
update_blocked_averages() in one go, this latency adds up quickly.
One noticeable factor in Joel's system is the presence of a lot of cgroups.
Which is essentially what makes __update_blocked_fair() expensive, and it seems
to always return something has decayed so we end up with a call to
sugov_update_shared() in every call.
I think we should limit the expensive call to update_blocked_averages() but
I honestly don't know what would be the right way to do it :-/
Or maybe there's another better alternative too..
Thanks
--
Qais Yousef
>
> > return true;
> >
> > update_blocked_averages(cpu);
> > --
> > 2.30.0.280.ga3ce27912f-goog
> >
next prev parent reply other threads:[~2021-01-22 19:35 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-22 15:46 [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ Joel Fernandes (Google)
2021-01-22 16:56 ` Vincent Guittot
2021-01-22 18:39 ` Qais Yousef [this message]
2021-01-22 19:14 ` Joel Fernandes
2021-01-25 13:23 ` Vincent Guittot
2021-01-26 16:36 ` Qais Yousef
2021-01-22 19:10 ` Joel Fernandes
2021-01-25 10:44 ` Dietmar Eggemann
2021-01-25 17:30 ` Vincent Guittot
2021-01-25 17:53 ` Dietmar Eggemann
2021-01-25 14:42 ` Vincent Guittot
2021-01-27 18:43 ` Joel Fernandes
2021-01-28 13:57 ` Vincent Guittot
2021-01-28 15:09 ` Joel Fernandes
2021-01-28 16:57 ` Qais Yousef
[not found] ` <CAKfTPtBvwm9vZb5C=2oTF6N-Ht6Rvip4Lv18yi7O3G8e-_ZWdg@mail.gmail.com>
2021-01-29 17:27 ` Vincent Guittot
2021-02-03 11:54 ` Dietmar Eggemann
2021-02-03 13:12 ` Vincent Guittot
2021-02-04 9:47 ` Dietmar Eggemann
2021-02-03 17:09 ` Qais Yousef
2021-02-03 17:35 ` Vincent Guittot
2021-02-04 10:45 ` Qais Yousef
2021-02-03 19:56 ` Joel Fernandes
2021-03-23 21:37 ` Tim Chen
2021-03-24 13:44 ` Vincent Guittot
2021-03-24 16:05 ` Tim Chen
2021-04-07 14:02 ` Vincent Guittot
2021-04-07 17:19 ` Tim Chen
2021-04-08 14:51 ` Vincent Guittot
2021-04-08 23:05 ` Tim Chen
2021-04-09 15:26 ` Vincent Guittot
2021-04-09 17:59 ` Tim Chen
2021-05-10 21:59 ` Tim Chen
2021-05-11 15:25 ` Vincent Guittot
2021-05-11 17:25 ` Tim Chen
2021-05-11 17:56 ` Vincent Guittot
2021-05-12 13:59 ` Qais Yousef
2021-05-13 18:45 ` Tim Chen
2021-05-17 16:14 ` Qais Yousef
2021-06-11 20:00 ` Tim Chen
2021-06-18 10:28 ` Vincent Guittot
2021-06-18 16:14 ` Tim Chen
2021-06-25 8:50 ` Vincent Guittot
2021-02-01 15:13 ` Joel Fernandes
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