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

[Mathieu Desnoyers]

On 9/12/23 10:14, Chen Yu wrote:
> On 2023-09-12 at 10:06:27 -0400, Mathieu Desnoyers wrote:
[...]
> > One more tweak: given that more than one task can update the cache_hot_timeout forward
> > one after another, and given that some tasks have larger burst_sleep_avg values than
> > others, I suspect we want to keep the forward movement monotonic with something like:
> >
> > if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running && p->se.burst_sleep_avg &&
> >      rq->cache_hot_timeout < now + p->se.burst_sleep_avg)
> > 	rq->cache_hot_timeout = now + p->se.burst_sleep_avg;
>
> Yeah, Aaron has mentioned this too:
> https://lore.kernel.org/lkml/ZP7SYu+gxlc%2FYjHu@chenyu5-mobl2/
> May I know the benefit of keeping forward movement monotonic?
> I thought that, should we only honor the latest dequeued task's burst_sleep_avg?
> Because we don't know whether the old deuqued task's cache has been scribbled by the latest
> dequeued task or not, does it still make sense to wake up the old dequeued task on its
> previous CPU?

Here is my reasoning:

If a second task is scheduled after the first dequeued task (a
task with large burst_sleep_avg) is dequeued, that second task (with
small burst_sleep_avg) would need to entirely scribble the other task's
cache lines within the time given by sysctl_sched_migration_cost, which
I suspect is typically not very large. So I doubt that the second task
can entirely kick out the first task cache lines within that time frame,
and therefore that second task should not move the cache_hot_timeout
value backwards.

But perhaps I'm missing something ?

Thanks,

Mathieu


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