Re: [PATCH v3 4/8] memcg: replace stats_flush_lock with an atomic

["Michal Koutný" <mkoutny-IBi9RG/b67k@public.gmane.org>]

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Hello.

On Thu, Mar 30, 2023 at 07:17:57PM +0000, Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>  static void __mem_cgroup_flush_stats(void)
>  {
> -	unsigned long flag;
> -
> -	if (!spin_trylock_irqsave(&stats_flush_lock, flag))
> +	/*
> +	 * We always flush the entire tree, so concurrent flushers can just
> +	 * skip. This avoids a thundering herd problem on the rstat global lock
> +	 * from memcg flushers (e.g. reclaim, refault, etc).
> +	 */
> +	if (atomic_read(&stats_flush_ongoing) ||
> +	    atomic_xchg(&stats_flush_ongoing, 1))
>  		return;

I'm curious about why this instead of

	if (atomic_xchg(&stats_flush_ongoing, 1))
		return;

Is that some microarchitectural cleverness?

Thanks,
Michal

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