Re: [PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle_cpu()

[Mathieu Desnoyers]

On 9/26/23 06:11, Chen Yu wrote:
> Problem statement:
> When task p is woken up, the scheduler leverages select_idle_sibling()
> to find an idle CPU for it. p's previous CPU is usually a preference
> because it can improve cache locality. However in many cases, the
> previous CPU has already been taken by other wakees, thus p has to
> find another idle CPU.
>
> Proposal:
> Inspired by Mathieu's idea[1], introduce the SIS_CACHE. It considers
> the sleep time of the task for better task placement. Based on the
> task's short sleeping history, keep p's previous CPU idle for a short
> while. Later when p is woken up, it can choose its previous CPU in
> select_idle_sibling(). When p's previous CPU is reserved, another wakee
> is not allowed to choose this CPU in select_idle_idle(). The reservation
> period is set to the task's average short sleep time, AKA, se->sis_rsv_avg.
>
> This does not break the work conservation of the scheduler, because
> wakee will still try its best to find an idle CPU. The difference is that
> different idle CPUs might have different priorities.
>
> Prateek pointed out that, with SIS_CACHE enabled, if all idle CPUs are
> cache-hot, select_idle_cpu() might have to choose a non-idle target CPU,
> which brings task stacking. Mitigate this by returning the first cache-hot
> idle CPU if no cache-cold CPU is found.

I've tried your patches on my reference hackbench workload:

./hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100

Unfortunately they don't appear to help for that specific load.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com