linux-kernel - Re: Hard lockup in blk_mq_free_request() / wbt_done() / wake_up

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Date:   Tue, 12 Jun 2018 17:19:57 +0100
From:   Chris Boot <bootc@....tc>
To:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: Hard lockup in blk_mq_free_request() / wbt_done() / wake_up_all()

On 12/06/18 17:09, Jens Axboe wrote:
> On 6/12/18 9:38 AM, Chris Boot wrote:
>> Hi folks,
>>
>> I maintain a large (to me) system with 112 threads (4x Intel E7-4830 v4)
>> which has a MegaRAID SAS 9361-24i controller. This system is currently
>> running Debian's 4.16.12 kernel (from stretch-backports) with blk_mq
>> enabled.
>>
>> I've run into a lockup which appears to involve blq_mq and writeback
>> throttling. It's hard to tell if I've run into this same thing with
>> older kernels; I'm trying to track down a deadlock but so far I've been
>> fairly certain that involved the OOM killer, but this doesn't seem to.
[snip]
> 
> Hmm that's really weird, I don't see how we could be spinning on the
> waitqueue lock like that. I haven't seen any wbt bug reports like this
> before.
> 
> Are things generally stable if you just turn off wbt? You can do that
> for sda, for instance, by doing:
> 
> # echo 0 > /sys/block/sda/queue/wbt_lat_usec
> 
> It'd be interesting to get this data point. Eg leave blk-mq enabled, and
> then just disable wbt.

Hi Jens,

Thanks for the speedy response. I'll see if I can get that tested soon;
if the system is stable without blk_mq I can see the users wanting to
keep it that way for a while. I'll let you know.

> Is anything disabling wbt in the system otherwise?

Not that I'm aware of, no.

Thanks,
Chris

-- 
Chris Boot
bootc@....tc