linux-kernel - Re: [PATCH -v2 15/17] sched: Fix migrate

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Message-ID: <20201006075939.GL2628@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Oct 2020 09:59:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
        qais.yousef@....com, swood@...hat.com, valentin.schneider@....com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com,
        tj@...nel.org
Subject: Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing

On Mon, Oct 05, 2020 at 04:57:32PM +0200, Peter Zijlstra wrote:
> +static inline struct task_struct *get_push_task(struct rq *rq)
> +{
> +	struct task_struct *p = rq->curr;
> +
> +	lockdep_assert_held(&rq->lock);
> +
> +	if (rq->push_busy)
> +		return NULL;
> +
> +	if (p->nr_cpus_allowed == 1)
> +		return NULL;

This; that means what when we're stuck below a per-cpu thread, we're
toast. There's just nothing much you can do... :/

> +
> +	rq->push_busy = true;
> +	return get_task_struct(p);
> +}