linux-kernel - Re: [PATCH] mm,page_alloc: PF_WQ_WORKER threads must sleep at should_reclaim

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Message-ID: <20180730144647.GX24267@dhcp22.suse.cz>
Date:   Mon, 30 Jul 2018 16:46:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Tejun Heo <tj@...nel.org>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm,page_alloc: PF_WQ_WORKER threads must sleep at
 should_reclaim_retry().

On Mon 30-07-18 23:34:23, Tetsuo Handa wrote:
> On 2018/07/30 18:32, Michal Hocko wrote:
[...]
> > This one is waiting for draining and we are in mm_percpu_wq WQ context
> > which has its rescuer so no other activity can block us for ever. So
> > this certainly shouldn't deadlock. It can be dead slow but well, this is
> > what you will get when your shoot your system to death.
> 
> We need schedule_timeout_*() to allow such WQ_MEM_RECLAIM workqueues to wake up. (Tejun,
> is my understanding correct?) Lack of schedule_timeout_*() does block WQ_MEM_RECLAIM
> workqueues forever.

Hmm. This doesn't match my understanding of what WQ_MEM_RECLAIM actually
guarantees. If you are right then the whole thing sounds quite fragile
to me TBH.

Anyway we would at least have an explanation for what you are seeing.
-- 
Michal Hocko
SUSE Labs