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SubjectRe: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting
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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

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