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Date:   Tue, 6 Oct 2020 09:59:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     tglx@linutronix.de, mingo@kernel.org
Cc:     linux-kernel@vger.kernel.org, bigeasy@linutronix.de,
        qais.yousef@arm.com, swood@redhat.com, valentin.schneider@arm.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com,
        tj@kernel.org
Subject: Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing
Message-ID: <20201006075939.GL2628@hirez.programming.kicks-ass.net>
References: <20201005145717.346020688@infradead.org>
 <20201005150922.458081448@infradead.org>
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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);
> +}