Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

Phil Auld
2024-11-05T15:20:23Z

On Tue, Nov 05, 2024 at 05:05:12AM +0100 Mike Galbraith wrote:
> On Mon, 2024-11-04 at 08:05 -0500, Phil Auld wrote:
> > On Sat, Nov 02, 2024 at 05:32:14AM +0100 Mike Galbraith wrote:
> >
> > >
> > > The buddy being preempted certainly won't be wakeup migrated...
> >
> > Not the waker who gets preempted but the wakee may be a bit more
> > sticky on his current cpu and thus stack more since he's still
> > in that runqueue.
>
> Ah, indeed, if wakees don't get scraped off before being awakened, they
> can and do miss chances at an idle CPU according to trace_printk().
>
> I'm undecided if overall it's boon, bane or even matters, as there is
> still an ample supply of wakeup migration, but seems it can indeed
> inject wakeup latency needlessly, so <sharpens stick>...
>
> My box booted and neither become exceptionally noisy nor inexplicably
> silent in.. oh, minutes now, so surely yours will be perfectly fine.
>
> After one minute of lightly loaded box browsing, trace_printk() said:
>
> 645 - racy peek says there is a room available
> 11 - cool, reserved room is free
> 206 - no vacancy or wakee pinned
> 38807 - SIS accommodates room seeker
>
> The below should improve the odds, but high return seems unlikely.
>

Thanks, I'll give it a spin with the nr_cpus_allowed bit.

Cheers,
Phil

> ---
> kernel/sched/core.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3790,7 +3790,13 @@ static int ttwu_runnable(struct task_str
> rq = __task_rq_lock(p, &rf);
> if (task_on_rq_queued(p)) {
> update_rq_clock(rq);
> - if (p->se.sched_delayed)
> + /*
> + * If wakee is mobile and the room it reserved is occupied, let it try to migrate.
> + */
> + if (p->se.sched_delayed && rq->nr_running > 1 && cpumask_weight(p->cpus_ptr) > 1) {
> + dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> + goto out_unlock;
> + } else if (p->se.sched_delayed)
> enqueue_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_DELAYED);
> if (!task_on_cpu(rq, p)) {
> /*
> @@ -3802,6 +3808,7 @@ static int ttwu_runnable(struct task_str
> ttwu_do_wakeup(p);
> ret = 1;
> }
> +out_unlock:
> __task_rq_unlock(rq, &rf);
>
> return ret;
>
>

--