Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Josef Bacik <jbacik@fb.com>
Cc: Chris Mason <clm@fb.com>, LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Dave Chinner <david@fromorbit.com>, Neil Brown <neilb@suse.de>,
	Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date: Fri, 11 Sep 2015 14:04:13 -0700	[thread overview]
Message-ID: <CA+55aFyK3Ua2ZcKLUBvESzxh=pH4x28QeijypJvSPiWbZStV-g@mail.gmail.com> (raw)
In-Reply-To: <55F33C2B.1010508@fb.com>

On Fri, Sep 11, 2015 at 1:40 PM, Josef Bacik <jbacik@fb.com> wrote:
>
> So we talked about this when we were trying to figure out a solution. The
> problem with this approach is now we have a plug that covers multiple super
> blocks (__writeback_inodes_wb loops through the sb's starts writeback),
> which is likely to give us crappier performance than no plug at all.

Why would that be? Either they are on separate disks, and the IO is
all independent anyway, and at most it got started by some (small)
CPU-amount later. Actual throughput should be the same. No?

Or the filesystems are on the same disk, in which case it should
presumably be a win to submit the IO together.

Of course, actual numbers would be the deciding factor if this really
is noticeable. But "cleaner code and saner locking" is definitely an
issue at least for me.

                Linus