Re: [PATCH 1/1] sched/fair: improve yield_to vs fairness

[Christian Borntraeger]

On 23.07.21 18:21, Mel Gorman wrote:
> On Fri, Jul 23, 2021 at 02:36:21PM +0200, Christian Borntraeger wrote:
> > > sched: Do not select highest priority task to run if it should be skipped
> > >
> > > <SNIP>
> > >
> > > index 44c452072a1b..ddc0212d520f 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4522,7 +4522,8 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > >    			se = second;
> > >    	}
> > > -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> > > +	if (cfs_rq->next &&
> > > +	    (cfs_rq->skip == left || wakeup_preempt_entity(cfs_rq->next, left) < 1)) {
> > >    		/*
> > >    		 * Someone really wants this to run. If it's not unfair, run it.
> > >    		 */
> >
> > I do see a reduction in ignored yields, but from a performance aspect for my
> > testcases this patch does not provide a benefit, while the the simple
> > 	curr->vruntime += sysctl_sched_min_granularity;
> > does.
>
> I'm still not a fan because vruntime gets distorted. From the docs
>
>     Small detail: on "ideal" hardware, at any time all tasks would have the same
>     p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
>     would ever get "out of balance" from the "ideal" share of CPU time
>
> If yield_to impacts this "ideal share" then it could have other
> consequences.
>
> I think your patch may be performing better in your test case because every
> "wrong" task selected that is not the yield_to target gets penalised and
> so the yield_to target gets pushed up the list.
>
> > I still think that your approach is probably the cleaner one, any chance to improve this
> > somehow?
>
> Potentially. The patch was a bit off because while it noticed that skip
> was not being obeyed, the fix was clumsy and isolated. The current flow is
>
> 1. pick se == left as the candidate
> 2. try pick a different se if the "ideal" candidate is a skip candidate
> 3. Ignore the se update if next or last are set
>
> Step 3 looks off because it ignores skip if next or last buddies are set
> and I don't think that was intended. Can you try this?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 44c452072a1b..d56f7772a607 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4522,12 +4522,12 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>   			se = second;
>   	}
>   
> -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> +	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, se) < 1) {
>   		/*
>   		 * Someone really wants this to run. If it's not unfair, run it.
>   		 */
>   		se = cfs_rq->next;
> -	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
> +	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, se) < 1) {
>   		/*
>   		 * Prefer last buddy, try to return the CPU to a preempted task.
>   		 */

This one alone does not seem to make a difference. Neither in ignored yield, nor
in performance.

Your first patch does really help in terms of ignored yields when
all threads are pinned to one host CPU. After that we do have no ignored yield
it seems. But it does not affect the performance of my testcase.
I did some more experiments and I removed the wakeup_preempt_entity checks in
pick_next_entity - assuming that this will result in source always being stopped
and target always being picked. But still, no performance difference.
As soon as I play with vruntime I do see a difference (but only without the cpu cgroup
controller). I will try to better understand the scheduler logic and do some more
testing. If you have anything that I should test, let me know.

Christian