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

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Date:   Tue, 15 Dec 2020 11:45:06 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        valentin.schneider@....com, qais.yousef@....com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, benbjiang@...il.com
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting

On Tue, Dec 15, 2020 at 08:59:11AM +0100, Peter Zijlstra wrote:
> On Tue, Dec 15, 2020 at 11:36:35AM +0800, Li, Aubrey wrote:
> > On 2020/12/15 0:48, Peter Zijlstra wrote:
> > > We compute the average cost of the total scan, but then use it as a
> > > per-cpu scan cost when computing the scan proportion. Fix this by
> > > properly computing a per-cpu scan cost.
> > > 
> > > This also fixes a bug where we would terminate early (!--nr, case) and
> > > not account that cost at all.
> > 
> > I'm a bit worried this may introduce a regression under heavy load.
> > The overhead of adding another cpu_clock() and calculation becomes 
> > significant when sis_scan is throttled by nr.
> 
> The thing is, the code as it exists today makes no sense what so ever.

Which makes it very hard to reason about or change in a "safe" manner as
all sorts of counter-intuitive effects occur.

The series is queued and running and takes 1-2 days. I haven't reviewed
the patches properly (holiday) but it'll be interesting to get some
provisional data at least.

-- 
Mel Gorman
SUSE Labs