linux-kernel - Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

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Message-ID: <20180130134049.GU2249@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Jan 2018 14:40:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>, rjw@...ysocki.net,
        srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle
 candidate and the basis for SIS

On Tue, Jan 30, 2018 at 01:25:27PM +0000, Mel Gorman wrote:
> > Because, esp. in this scenario; a task migrating; the hardware really
> > can't do anything sensible, whereas the OS _knows_.
> 
> Potentially yes. One option without HWP would be to track utilisation
> for a task or artifically boost it for a short period after migration so
> a higher p-state is potentially selected. With HWP, a hint would have to
> be given to the hardware to try select a higher frequency but I've no idea
> how expensive that is or how it would behave on different implementations
> of HWP. It may also be a game of whack-a-mole trying to get every cpufreq
> configuration correct.

We have much of this infrastructure and have been working on improving
it [1]. We already track per task utilization and feed it into a cpufreq
governor (schedutil).

> One advantage of using fewer cores is that it should
> work regardless of cpufreq driver.

Sure, and I started out by saying this patch isn't necessarily bad; but
I think our 'use' [2] of HWP _is_.


[1] https://lkml.kernel.org/r/20180123180847.4477-1-patrick.bellasi@arm.com
[2] IIRC we basically don't do _anything_ when HWP.