linux-kernel - Re: [RFC] mm, slab: reschedule cache

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Message-ID: <alpine.DEB.2.20.1804100907160.27333@nuc-kabylake>
Date:   Tue, 10 Apr 2018 09:12:08 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [RFC] mm, slab: reschedule cache_reap() on the same CPU

On Tue, 10 Apr 2018, Vlastimil Babka wrote:

> cache_reap() is initially scheduled in start_cpu_timer() via
> schedule_delayed_work_on(). But then the next iterations are scheduled via
> schedule_delayed_work(), thus using WORK_CPU_UNBOUND.

That is a bug.. cache_reap must run on the same cpu since it deals with
the per cpu queues of the current cpu. Scheduled_delayed_work() used to
guarantee running on teh same cpu.

> This patch makes sure schedule_delayed_work_on() is used with the proper cpu
> when scheduling the next iteration. The cpu is stored with delayed_work on a
> new slab_reap_work_struct super-structure.

The current cpu is readily available via smp_processor_id(). Why a
super structure?

> @@ -4074,7 +4086,8 @@ static void cache_reap(struct work_struct *w)
>  	next_reap_node();
>  out:
>  	/* Set up the next iteration */
> -	schedule_delayed_work(work, round_jiffies_relative(REAPTIMEOUT_AC));
> +	schedule_delayed_work_on(reap_work->cpu, work,
> +					round_jiffies_relative(REAPTIMEOUT_AC));

schedule_delayed_work_on(smp_processor_id(), work, round_jiffies_relative(REAPTIMEOUT_AC));

instead all of the other changes?