linux-kernel - Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle

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Message-ID: <X/i3TbRRS9chYb3l@hirez.programming.kicks-ass.net>
Date:   Fri, 8 Jan 2021 20:49:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Jiang Biao <benbjiang@...il.com>
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting

On Fri, Jan 08, 2021 at 04:10:51PM +0100, Vincent Guittot wrote:
> Another thing that worries me, is that we use the avg_idle of the
> local cpu, which is obviously not idle otherwise it would have been
> selected, to decide how much time we should spend on looking for
> another idle CPU. I'm not sure that's the right metrics to use
> especially with a possibly stalled value.

The thinking was that if this CPU has little idle time, this CPU should
not spend a lot of time searching...

That is; if we spend more time looking for places to run, than we have
idle time, we're loosing cycles we could've ran (supposedly useful) work.

The only counter argument is tail latency.