From mboxrd@z Thu Jan  1 00:00:00 1970
From: Artem Bityutskiy <dedekind1@gmail.com>
Subject: Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads
 wakeups
Date: Thu, 22 Jul 2010 16:27:00 +0300
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Cc: Dave Chinner <david@fromorbit.com>,
	Dave Chinner <dchinner@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>
To: Christoph Hellwig <hch@infradead.org>
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On Thu, 2010-07-22 at 05:00 -0400, Christoph Hellwig wrote:
> On Thu, Jul 22, 2010 at 10:41:55AM +1000, Dave Chinner wrote:
> > 	if (wakeup_bdi) {
> > 		trace_writeback_wakeup(bdi)
> > 		spin_lock(&bdi->wb_lock);
> > 		if (!bdi->wb.task) {{
> > 			trace_writeback_wakeup_nothread(bdi);
> > 			wake_up_process(default_backing_dev_info.wb.task);
> > 		} else
> > 			wake_up_process(bdi->wb.task);
> > 		spin_unlock(&bdi->wb_lock);
>=20
> That gives us duplicate traces for the first case, what about:
>=20
> 	 if (wakeup_bdi) {
> 	 	spin_lock(&bdi->wb_lock);
> 		if (bdi->wb.task) {
> 			trace_writeback_wake_thread(bdi);
> 			wake_up_process(bdi->wb.task);
> 		} else {
> 			trace_writeback_wake_forker_thread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		}
> 		spin_unlock(&bdi->wb_lock);
> 	}

But Dave's version is what we have in 'bdi_queue_work()' as well. I it
is OK - first trace point is triggered every time the bdi thread is
_wanted_ to be woken up.

Then if it does not exist, we need to do something special to wake it u=
p
(ask the forker thread which to create it). This is a different event
and we use a different trace point.

--=20
Best Regards,
Artem Bityutskiy (=D0=90=D1=80=D1=82=D1=91=D0=BC =D0=91=D0=B8=D1=82=D1=8E=
=D1=86=D0=BA=D0=B8=D0=B9)

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