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Date:   Thu, 26 Nov 2020 10:19:38 +1100
From:   Balbir Singh <bsingharora@gmail.com>
To:     Joel Fernandes <joel@joelfernandes.org>
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        Julien Desfossez <jdesfossez@digitalocean.com>,
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Subject: Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick_task()
Message-ID: <20201125231938.GE163610@balbir-desktop>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-3-joel@joelfernandes.org>
 <025bb649-89c5-a353-1434-2da725adffd9@gmail.com>
 <20201120165854.GB1021337@google.com>
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On Fri, Nov 20, 2020 at 11:58:54AM -0500, Joel Fernandes wrote:
> 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")
>

I did see the next patch, but the idea is that we don't need to
update_curr() if the picked sched entity is the same as current (se ==
curr). What are the side-effects of not updating curr when se == curr?

Balbir