linux-kernel - Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish

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Message-ID: <20150917021453.GO3902@dastard>
Date:	Thu, 17 Sep 2015 12:14:53 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Mason <clm@...com>, Jan Kara <jack@...e.cz>,
	Josef Bacik <jbacik@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

On Wed, Sep 16, 2015 at 06:12:29PM -0700, Linus Torvalds wrote:
> On Wed, Sep 16, 2015 at 5:37 PM, Dave Chinner <david@...morbit.com> wrote:
> >
> > TL;DR: Results look really bad - not only is the plugging
> > problematic, baseline writeback performance has regressed
> > significantly.
> 
> Dave, if you're testing my current -git, the other performance issue
> might still be the spinlock thing.

I have the fix as the first commit in my local tree - it'll remain
there until I get a conflict after an update. :)

> The plugging IO pauses are interesting, though. Plugging really
> *shouldn't* cause that kind of pauses, _regardless_ of what level it
> happens on, so I wonder if the patch ends up just exposing some really
> basic problem that just normally goes hidden.

Right, that's what I suspect - it didn't happen on older kernels,
but we've just completely reworked the writeback code for the
control group awareness since I last looked really closely at
this...

> Can you match up the IO wait times with just *where* it is
> waiting? Is it waiting for that inode I_SYNC thing in
> inode_sleep_on_writeback()?

I'll do some more investigation.

> But it does sound like we should just revert the whole plugging for
> now, if only because "it has odd effects".

Yup - we can add it again next merge window once we get to the
bottom of whatever is going on and have had time to test the new
code properly.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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