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Date: Tue, 10 Apr 2018 09:12:08 -0500 (CDT)
From: Christopher Lameter <cl@linux.com>
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To: Vlastimil Babka <vbabka@suse.cz>
cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>, Tejun Heo <tj@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>, Stephen Boyd <sboyd@kernel.org>
Subject: Re: [RFC] mm, slab: reschedule cache_reap() on the same CPU
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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?