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Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
To: Chris Mason <clm@fb.com>, Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>,
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From: Jens Axboe <axboe@fb.com>
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Date: Fri, 18 Sep 2015 08:23:46 -0600
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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