linux-kernel - Re: [PATCH -v2 03/17] sched/hotplug: Ensure only per-cpu kthreads run during hotplug

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Date:   Tue, 6 Oct 2020 09:25:40 +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 03/17] sched/hotplug: Ensure only per-cpu kthreads
 run during hotplug

On Mon, Oct 05, 2020 at 04:57:20PM +0200, Peter Zijlstra wrote:
> +static inline void balance_switch(struct rq *rq)
> +{
> +	if (unlikely(rq->balance_flags)) {
> +		/*
> +		 * Run the balance_callbacks, except on hotplug
> +		 * when we need to push the current task away.
> +		 */
> +		if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) ||
> +		    !(rq->balance_flags & BALANCE_PUSH) ||
> +		    !balance_push(rq))
> +			__balance_callbacks(rq);
> +	}
> +}

> @@ -1392,12 +1396,13 @@ queue_balance_callback(struct rq *rq,
>  {
>  	lockdep_assert_held(&rq->lock);
>  
> -	if (unlikely(head->next))
> +	if (unlikely(head->next || (rq->balance_flags & BALANCE_PUSH)))
>  		return;

With this bit from Valentin we can probably simplify the above function,
but I've not thought about that yet.