linux-kernel - Re: [PATCH 2/7] sched/fair: Add group_misfit

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Date:   Mon, 19 Feb 2018 14:56:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     mingo@...hat.com, valentin.schneider@....com,
        dietmar.eggemann@....com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] sched/fair: Add group_misfit_task load-balance type

On Thu, Feb 15, 2018 at 04:20:49PM +0000, Morten Rasmussen wrote:
> @@ -6733,9 +6758,12 @@ done: __maybe_unused
>  	if (hrtick_enabled(rq))
>  		hrtick_start_fair(rq, p);
>  
> +	update_misfit_status(p, rq);
> +
>  	return p;
>  
>  idle:
> +	update_misfit_status(NULL, rq);
>  	new_tasks = idle_balance(rq, rf);
>  
>  	/*

So we set a point when picking a task (or tick). We clear said pointer
when idle.

> @@ -7822,6 +7855,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  		 */
>  		if (!nr_running && idle_cpu(i))
>  			sgs->idle_cpus++;
> +
> +		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> +		    !sgs->group_misfit_task_load && rq->misfit_task_load)
> +			sgs->group_misfit_task_load = rq->misfit_task_load;
>  	}
>  
>  	/* Adjust by relative CPU capacity of the group */

And we read said pointer from another CPU, without holding the
respective rq->lock.

What happens, if right after we set sgs->group_misfit_task_load, our
task decides to exit?