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

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Message-ID: <20201215075911.GA3040@hirez.programming.kicks-ass.net>
Date:   Tue, 15 Dec 2020 08:59:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     mgorman@...hsingularity.net, 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 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.
It's plain broken batshit.

We calculate the total scanning time (irrespective of how many CPUs we
touched), and then use that calculate the number of cpus to scan. That's
just daft.

After this patch we calculate the avg cost of scanning 1 cpu and use
that to calculate how many cpus to scan. Which is coherent and sane.

Maybe it can be improved, but that's a completely different thing.