linux-kernel - Re: [RFC PATCH sched/urgent] sched: Task still delay-dequeued after switched from fair

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Message-ID: <ZyKUsBIXHu3NIcUb@slm.duckdns.org>
Date: Wed, 30 Oct 2024 10:18:56 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, David Vernet <void@...ifault.com>,
	sched-ext@...a.com
Subject: Re: [RFC PATCH sched/urgent] sched: Task still delay-dequeued after
 switched from fair

Hello,

On Wed, Oct 30, 2024 at 11:49:34AM +0100, Peter Zijlstra wrote:
...
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > index 65334c13ffa5..601aad1a2625 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -5205,8 +5205,12 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
> >  	while ((p = scx_task_iter_next_locked(&sti))) {
> >  		const struct sched_class *old_class = p->sched_class;
> >  		struct sched_enq_and_set_ctx ctx;
> > +		int deq_flags = DEQUEUE_SAVE | DEQUEUE_MOVE;
> >  
> > -		sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE, &ctx);
> > +		if (p->se.sched_delayed)
> > +			deq_flags |= DEQUEUE_SLEEP | DEQUEUE_DELAYED;
> > +
> > +		sched_deq_and_put_task(p, deq_flags, &ctx);
> 
> I don't think this is quite right, the problem is that in this case
> ctx.queued is reporting true, even though you want it false.
> 
> This is why 98442f0ccd82 ("sched: Fix delayed_dequeue vs switched_from_fair()")
> adds a second dequeue.

I see. Yeah, ctx.queued would be set incorrectly.

> Also, you seem to have a second instance of all that.

The disable path doesn't really need it because the transition direction is
always scx -> fair but yeah keeping the two loops in sync is fine too.

> Does the below work for you? I suppose I might as well go work on that
> TODO item now.

Yeap, it works. Will ack on the other thread.

Thanks.

-- 
tejun