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Message-ID: <20180730154424.GG1206094@devbig004.ftw2.facebook.com>
Date: Mon, 30 Jul 2018 08:44:24 -0700
From: Tejun Heo <tj@...nel.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Michal Hocko <mhocko@...nel.org>, 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().
Hello,
On Tue, Jul 31, 2018 at 12:25:04AM +0900, Tetsuo Handa wrote:
> WQ_MEM_RECLAIM guarantees that "struct task_struct" is preallocated. But
> WQ_MEM_RECLAIM does not guarantee that the pending work is started as soon
> as an item was queued. Same rule applies to both WQ_MEM_RECLAIM workqueues
> and !WQ_MEM_RECLAIM workqueues regarding when to start a pending work (i.e.
> when schedule_timeout_*() is called).
>
> Is this correct?
WQ_MEM_RECLAIM guarantees that there's always gonna exist at least one
kworker running the workqueue. But all per-cpu kworkers are subject
to concurrency limiting execution - ie. if there are any per-cpu
actively running on a cpu, no futher kworkers will be scheduled.
> > We can add timeout mechanism to workqueue so that it
> > kicks off other kworkers if one of them is in running state for too
> > long, but idk, if there's an indefinite busy loop condition in kernel
> > threads, we really should get rid of them and hung task watchdog is
> > pretty effective at finding these cases (at least with preemption
> > disabled).
>
> Currently the page allocator has a path which can loop forever with
> only cond_resched().
Yeah, workqueue can choke on things like that and kthread indefinitely
busy looping doesn't do anybody any good.
Thanks.
--
tejun
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