Re: [PATCH 00/17] sched: EEVDF using latency-nice

[Hillf Danton <hdanton@sina.com>]

On Tue, 11 Apr 2023 12:15:41 +0200 Mike Galbraith <efault@gmx.de>
> On Mon, 2023-04-10 at 16:23 +0800, Hillf Danton wrote:
> >
> > In order to only narrow down the poor performance reported, make a tradeoff
> > between runtime and latency simply by restoring sysctl_sched_min_granularity
> > at tick preempt, given the known order on the runqueue.
> 
> Tick preemption isn't the primary contributor to the scheduling delta,
> it's wakeup preemption. If you look at the perf summaries of 5 minute
> recordings on my little 8 rq box below, you'll see that the delta is
> more than twice what a 250Hz tick could inflict.  You could also just
> turn off WAKEUP_PREEMPTION and watch the delta instantly peg negative.
> 
> Anyway...
> 
> Given we know preemption is markedly up, and as always a source of pain
> (as well as gain), perhaps we can try to tamp it down a little without
> inserting old constraints into the shiny new scheduler.
> 
> The dirt simple tweak below puts a dent in the sting by merely sticking
> with whatever decision EEVDF last made until it itself invalidates that
> decision. It still selects via the same math, just does so the tiniest
> bit less frenetically.
> 
> ---
>  kernel/sched/fair.c     |    3 +++
>  kernel/sched/features.h |    6 ++++++
>  2 files changed, 9 insertions(+)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -950,6 +950,9 @@ static struct sched_entity *pick_eevdf(s
>  	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
>  		curr =3D NULL;
> 
> +	if (sched_feat(GENTLE_EEVDF) && curr)
> +		return curr;
> +

This is rather aggressive, given latency-10 curr and latency-0 candidate
at tick hit for instance. And along your direction a mild change is
postpone the preempt wakeup to the next tick.

+++ b/kernel/sched/fair.c
@@ -7932,8 +7932,6 @@ static void check_preempt_wakeup(struct
 		return;
 
 	cfs_rq = cfs_rq_of(se);
-	update_curr(cfs_rq);
-
 	/*
 	 * XXX pick_eevdf(cfs_rq) != se ?
 	 */