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Subject: Re: [PATCH] blk-wbt: Avoid lock contention and thundering herd issue
 in wbt_wait
From:   Jens Axboe <axboe@kernel.dk>
To:     =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= <holger@applied-asynchrony.com>,
        Anchal Agarwal <anchalag@amazon.com>
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        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
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Date:   Wed, 22 Aug 2018 11:28:30 -0600
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On 8/22/18 8:27 AM, Jens Axboe wrote:
> On 8/22/18 6:54 AM, Holger Hoffstätte wrote:
>> On 08/22/18 06:10, Jens Axboe wrote:
>>> [...]
>>> If you have time, please look at the 3 patches I posted earlier today.
>>> Those are for mainline, so should be OK :-)
>>
>> I'm just playing along at home but with those 3 I get repeatable
>> hangs & writeback not starting at all, but curiously *only* on my btrfs
>> device; for inexplicable reasons some other devices with ext4/xfs flush
>> properly. Yes, that surprised me too, but it's repeatable.
>> Now this may or may not have something to do with some of my in-testing
>> patches for btrfs itself, but if I remove those 3 wbt fixes, everything
>> is golden again. Not eager to repeat since it hangs sync & requires a
>> hard reboot.. :(
>> Just thought you'd like to know.
> 
> Thanks, that's very useful info! I'll see if I can reproduce that.

Any chance you can try with and see which patch is causing the issue?
I can't reproduce it here, seems solid.

Either that, or a reproducer would be great...

-- 
Jens Axboe