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

From: Jens Axboe <axboe@fb.com>
To: Chris Mason <clm@fb.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>,
	Josef Bacik <jbacik@fb.com>, LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Neil Brown <neilb@suse.de>, Christoph Hellwig <hch@lst.de>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date: Fri, 18 Sep 2015 08:23:46 -0600	[thread overview]
Message-ID: <55FC1E72.3040500@fb.com> (raw)
In-Reply-To: <20150918131615.GI8624@ret.masoncoding.com>

On 09/18/2015 07:16 AM, Chris Mason wrote:
> On Thu, Sep 17, 2015 at 11:04:03PM -0700, Linus Torvalds wrote:
>> On Thu, Sep 17, 2015 at 10:40 PM, Dave Chinner <david@fromorbit.com> wrote:
>>>
>>> Ok, makes sense - the plug is not being flushed as we switch away,
>>> but Chris' patch makes it do that.
>>
>> Yup.
>
> Huh, that does make much more sense, thanks Linus.  I'm wondering where
> else I've assumed that cond_resched() unplugged.
>
>>
>> And I actually think Chris' patch is better than the one I sent out
>> (but maybe the scheduler people should take a look at the behavior of
>> cond_resched()), I just wanted you to test that to verify the
>> behavior.

It makes no sense for preemption schedule to NOT unplug, the fact that 
it doesn't is news to me as well. It was never the intent of the 
unplug-on-schedule to NOT unplug for certain schedule out events, that 
seems like very odd behavior.

-- 
Jens Axboe