Date: Mon, 8 Dec 2014 12:40:52 -0500
From: Tejun Heo <tj@kernel.org>
To: NeilBrown <neilb@suse.de>
Cc: Jan Kara <jack@suse.cz>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Dongsu Park <dongsu.park@profitbricks.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH workqueue/for-3.18-fixes 2/2] workqueue: allow rescuer
 thread to do more work
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On Thu, Dec 04, 2014 at 10:12:23AM -0500, Tejun Heo wrote:
> From: NeilBrown <neilb@suse.de>
> 
> When there is serious memory pressure, all workers in a pool could be
> blocked, and a new thread cannot be created because it requires memory
> allocation.
> 
> In this situation a WQ_MEM_RECLAIM workqueue will wake up the
> rescuer thread to do some work.
> 
> The rescuer will only handle requests that are already on ->worklist.
> If max_requests is 1, that means it will handle a single request.
> 
> The rescuer will be woken again in 100ms to handle another max_requests
> requests.
> 
> I've seen a machine (running a 3.0 based "enterprise" kernel) with
> thousands of requests queued for xfslogd, which has a max_requests of
> 1, and is needed for retiring all 'xfs' write requests.  When one of
> the worker pools gets into this state, it progresses extremely slowly
> and possibly never recovers (only waited an hour or two).
> 
> With this patch we leave a pool_workqueue on mayday list
> until it is clearly no longer in need of assistance.  This allows
> all requests to be handled in a timely fashion.
> 
> We keep each pool_workqueue on the mayday list until
> need_to_create_worker() is false, and no work for this workqueue is
> found in the pool.
> 
> I have tested this in combination with a (hackish) patch which forces
> all work items to be handled by the rescuer thread.  In that context
> it significantly improves performance.  A similar patch for a 3.0
> kernel significantly improved performance on a heavy work load.
> 
> Thanks to Jan Kara for some design ideas, and to Dongsu Park for
> some comments and testing.
> 
> tj: Inverted the lock order between wq_mayday_lock and pool->lock with
>     a preceding patch and simplified this patch.  Added comment and
>     updated changelog accordingly.  Dongsu spotted missing get_pwq()
>     in the simplified code.
> 
> Cc: Dongsu Park <dongsu.park@profitbricks.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> Signed-off-by: NeilBrown <neilb@suse.de>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Too late for for-3.18-fixes.  Applied the two patches to wq/for-3.19.

Thanks.

-- 
tejun
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