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

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Date:   Tue, 15 Dec 2020 20:13:07 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 2020/12/15 15:59, 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.
> 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.

I see and all of these make sense to me.

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

OK, I'll go through the workloads in hand and paste the data here.

Thanks,
-Aubrey