linux-kernel - Re: [PATCH workqueue/for-3.18-fixes 2/2] workqueue: allow rescuer thread to do more work

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Date:	Tue, 9 Dec 2014 09:47:35 +1100
From:	NeilBrown <neilb@...e.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, Lai Jiangshan <laijs@...fujitsu.com>,
	Dongsu Park <dongsu.park@...fitbricks.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH workqueue/for-3.18-fixes 2/2] workqueue: allow rescuer
 thread to do more work

On Mon, 8 Dec 2014 12:40:52 -0500 Tejun Heo <tj@...nel.org> wrote:

> On Thu, Dec 04, 2014 at 10:12:23AM -0500, Tejun Heo wrote:
> > From: NeilBrown <neilb@...e.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@...fitbricks.com>
> > Cc: Jan Kara <jack@...e.cz>
> > Cc: Lai Jiangshan <laijs@...fujitsu.com>
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > Signed-off-by: Tejun Heo <tj@...nel.org>
> 
> Too late for for-3.18-fixes.  Applied the two patches to wq/for-3.19.

I've just run my synthetic test which forces most works to be handled by the
rescuer thread.  With these patches it works more smoothly than without.
And importantly: it still works :-)

Thanks,
NeilBrown

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