linux-kernel - Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick

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Message-ID: <20201120165854.GB1021337@google.com>
Date:   Fri, 20 Nov 2020 11:58:54 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Singh, Balbir" <bsingharora@...il.com>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
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Subject: Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick_task()

On Fri, Nov 20, 2020 at 10:56:09AM +1100, Singh, Balbir wrote:
[..] 
> > +#ifdef CONFIG_SMP
> > +static struct task_struct *pick_task_fair(struct rq *rq)
> > +{
> > +	struct cfs_rq *cfs_rq = &rq->cfs;
> > +	struct sched_entity *se;
> > +
> > +	if (!cfs_rq->nr_running)
> > +		return NULL;
> > +
> > +	do {
> > +		struct sched_entity *curr = cfs_rq->curr;
> > +
> > +		se = pick_next_entity(cfs_rq, NULL);
> > +
> > +		if (curr) {
> > +			if (se && curr->on_rq)
> > +				update_curr(cfs_rq);
> > +
> > +			if (!se || entity_before(curr, se))
> > +				se = curr;
> > +		}
> 
> Do we want to optimize this a bit 
> 
> if (curr) {
> 	if (!se || entity_before(curr, se))
> 		se = curr;
> 
> 	if ((se != curr) && curr->on_rq)
> 		update_curr(cfs_rq);
> 
> }

Do you see a difference in codegen? What's the optimization?

Also in later patches this code changes, so it should not matter:
See: 3e0838fa3c51 ("sched/fair: Fix pick_task_fair crashes due to empty rbtree")

thanks,

 - Joel