linux-kernel - Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load

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Message-ID: <20160331104705.GG3430@twins.programming.kicks-ass.net>
Date:	Thu, 31 Mar 2016 12:47:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()

On Thu, Mar 31, 2016 at 11:50:40AM +0200, Vincent Guittot wrote:
> > In what case? Assuming you talk about a remove wakeup, no. Only if that
> > wakeup results in a preemption, which isn't a given.
> 
> yes, i was speaking about a remote wakeup.
> In the ttwu_queue_remote,  there is a call to smp_send_reschedule. Is
> there another way to add a remote task in the wake list ?

Right, but at that point we don't yet know how much util will change.

And doing that IPI unconditionally is expensive; see:

  518cd6234178 ("sched: Only queue remote wakeups when crossing cache boundaries")

13% regression on TCP_RR.