linux-kernel - Re: [PATCH 3/4] mm, page_alloc: Drain per-cpu pages from workqueue context

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Message-ID: <20170123200412.mkesardc4mckk6df@techsingularity.net>
Date:   Mon, 23 Jan 2017 20:04:12 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Tejun Heo <tj@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Petr Mladek <pmladek@...e.cz>
Subject: Re: [PATCH 3/4] mm, page_alloc: Drain per-cpu pages from workqueue
 context

On Mon, Jan 23, 2017 at 12:03:29PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Fri, Jan 20, 2017 at 03:26:06PM +0000, Mel Gorman wrote:
> > > This translates to queue_work_on(), which has the comment of "We queue
> > > the work to a specific CPU, the caller must ensure it can't go away.",
> > > so is this safe? lru_add_drain_all() uses get_online_cpus() around this.
> > > 
> > 
> > get_online_cpus() would be required.
> 
> This part of workqueue usage has always been a bit clunky and I should
> imrpove it but you don't necessarily have to pin the cpus from
> queueing to execution.  You can queue without checking whether the CPU
> is online and instead synchronize the actual work item execution
> against cpu offline callback so that if the work item gets executed
> after offline callback is finished, it becomes a noop.
> 

What is the actual mechanism that does that? It's not something that
schedule_on_each_cpu does and one would expect that the core workqueue
implementation would get this sort of detail correct. Or is this a proposal
on how it should be done?

-- 
Mel Gorman
SUSE Labs