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Date:   Mon, 30 Jul 2018 08:44:24 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm,page_alloc: PF_WQ_WORKER threads must sleep at
 should_reclaim_retry().
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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