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From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Steve Muckle <steve.muckle@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <Juri.Lelli@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Michael Turquette <mturquette@baylibre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()
Date: Thu, 31 Mar 2016 11:34:08 +0200	[thread overview]
Message-ID: <20160331093408.GB12845@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKfTPtCPfd2-jGDn-9ZLjNKTni4czuvrkH7KFh5L2EtDeTQE5A@mail.gmail.com>

On Thu, Mar 31, 2016 at 11:27:22AM +0200, Vincent Guittot wrote:
> On 30 March 2016 at 21:35, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Mar 28, 2016 at 12:38:26PM -0700, Steve Muckle wrote:
> >> Without covering all the paths where CFS utilization changes it's
> >> possible to have to wait up to a tick to act on some changes, since the
> >> tick is the only guaranteed regularly-occurring instance of the hook.
> >> That's an unacceptable amount of latency IMO...
> >
> > Note that even with your patches that might still be the case. Remote
> > wakeups might not happen on the destination CPU at all, so it might not
> > be until the next tick (which always happens locally) that we'll
> > 'observe' the utilization change brought with the wakeups.
> >
> > We could force all the remote wakeups to IPI the destination CPU, but
> > that comes at a significant performance cost.
> 
> Isn't a reschedule ipi already sent in this case ?

In what case? Assuming you talk about a remove wakeup, no. Only if that
wakeup results in a preemption, which isn't a given.

And we really don't want to carry the 'has util increased' information
all the way down to where we make that decision.

  reply	other threads:[~2016-03-31  9:34 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-22  0:21 [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg() Steve Muckle
2016-03-22  0:21 ` [PATCH 2/2] sched/fair: do not call cpufreq hook unless util changed Steve Muckle
2016-03-24 23:47   ` Sai Gurrappadi
2016-03-25  1:01     ` Steve Muckle
2016-03-25 21:24       ` Sai Gurrappadi
2016-04-23 12:57   ` [tip:sched/core] sched/fair: Do " tip-bot for Steve Muckle
2016-03-28 12:02 ` [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg() Dietmar Eggemann
2016-03-28 16:34   ` Steve Muckle
2016-03-28 18:30     ` Dietmar Eggemann
2016-03-28 19:38       ` Steve Muckle
2016-03-30 19:35         ` Peter Zijlstra
2016-03-31  1:42           ` Steve Muckle
2016-03-31  7:37             ` Peter Zijlstra
2016-03-31 21:26               ` Steve Muckle
2016-04-01  9:20                 ` Peter Zijlstra
2016-04-11 19:28                   ` Steve Muckle
2016-04-11 21:20                     ` Rafael J. Wysocki
2016-04-12 14:29                       ` Rafael J. Wysocki
2016-04-12 19:38                         ` Steve Muckle
2016-04-13 14:45                           ` Rafael J. Wysocki
2016-04-13 17:53                             ` Steve Muckle
2016-04-13 19:39                               ` Rafael J. Wysocki
2016-04-13  0:08                         ` Steve Muckle
2016-04-13  4:48                           ` Rafael J. Wysocki
2016-04-13 16:05                             ` Rafael J. Wysocki
2016-04-13 16:07                               ` Rafael J. Wysocki
2016-04-13 18:06                                 ` Steve Muckle
2016-04-13 19:50                                   ` Rafael J. Wysocki
2016-04-20  2:22                                     ` Steve Muckle
2016-03-31  9:27           ` Vincent Guittot
2016-03-31  9:34             ` Peter Zijlstra [this message]
2016-03-31  9:50               ` Vincent Guittot
2016-03-31 10:47                 ` Peter Zijlstra
2016-03-31 12:14                   ` Vincent Guittot
2016-03-31 12:34                     ` Peter Zijlstra
2016-03-31 12:50                       ` Vincent Guittot
2016-04-23 12:57 ` [tip:sched/core] sched/fair: Move " tip-bot for Steve Muckle

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