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Date:   Fri, 26 Apr 2019 16:02:09 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vineeth Remanan Pillai <vpillai@digitalocean.com>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v2 09/17] sched: Introduce sched_class::pick_task()
Message-ID: <20190426140209.GA2623@hirez.programming.kicks-ass.net>
References: <cover.1556025155.git.vpillai@digitalocean.com>
 <e2f1d30b0b3bb62c01824f422badf159147982d7.1556025155.git.vpillai@digitalocean.com>
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On Tue, Apr 23, 2019 at 04:18:14PM +0000, Vineeth Remanan Pillai wrote:
> +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 (!(se || curr))
> +			return NULL;

I didn't get around to reading the original discussion here, but how can
that possibly be?

I can see !se, in that case curr is still selected.

I can also see !curr, in that case curr is put.

But I cannot see !se && !curr, per the above check we know
cfs_rq->nr_running != 0, so there must be a cfs task to find. This means
either curr or se must exist.

> +
> +		if (curr) {
> +			if (se && curr->on_rq)
> +				update_curr(cfs_rq);
> +
> +			if (!se || entity_before(curr, se))
> +				se = curr;
> +		}
> +
> +		cfs_rq = group_cfs_rq(se);
> +	} while (cfs_rq);
> +
> +	return task_of(se);
> +}