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

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Message-ID: <20170123205501.GA25944@htj.duckdns.org>
Date:   Mon, 23 Jan 2017 15:55:01 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
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

Hello, Mel.

On Mon, Jan 23, 2017 at 08:04:12PM +0000, Mel Gorman wrote:
> 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?

If you use schedule_on_each_cpu(), it's all fine as the thing pins
cpus and waits for all the work items synchronously.  If you wanna do
it asynchronously, right now, you'll have to manually synchronize work
items against the offline callback manually.

On this area, the current workqueue behavior is pretty bad.
Historically, we didn't distinguish affinity-for-optimization
affinity-for-correctness, so we couldn't really enforce strong
behaviors on it.  We started distinguishing them some releases ago, so
I should revisit it soon.

Thanks.

--
tejun