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

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Message-ID: <20201125231938.GE163610@balbir-desktop>
Date:   Thu, 26 Nov 2020 10:19:38 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
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,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
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        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick_task()

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