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Date:   Wed, 17 Aug 2022 13:49:33 +0200
From:   Jan Kara <jack@suse.cz>
To:     Holger =?utf-8?Q?Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>
Cc:     Chris Murphy <lists@colorremedies.com>,
        Nikolay Borisov <nborisov@suse.com>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Paolo Valente <paolo.valente@linaro.org>,
        Linux-RAID <linux-raid@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Josef Bacik <josef@toxicpanda.com>
Subject: Re: stalling IO regression since linux 5.12, through 5.18
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On Wed 17-08-22 11:52:54, Holger Hoffst?tte wrote:
> On 2022-08-16 17:34, Chris Murphy wrote:
> > 
> > On Tue, Aug 16, 2022, at 11:25 AM, Nikolay Borisov wrote:
> > > How about changing the scheduler either mq-deadline or noop, just
> > > to see if this is also reproducible with a different scheduler. I
> > > guess noop would imply the blk cgroup controller is going to be
> > > disabled
> > 
> > I already reported on that: always happens with bfq within an hour or
> > less. Doesn't happen with mq-deadline for ~25+ hours. Does happen
> > with bfq with the above patches removed. Does happen with
> > cgroup.disabled=io set.
> > 
> > Sounds to me like it's something bfq depends on and is somehow
> > becoming perturbed in a way that mq-deadline does not, and has
> > changed between 5.11 and 5.12. I have no idea what's under bfq that
> > matches this description.
> 
> Chris, just a shot in the dark but can you try the patch from
> 
> https://lore.kernel.org/linux-block/20220803121504.212071-1-yukuai1@huaweicloud.com/
> 
> on top of something more recent than 5.12? Ideally 5.19 where it applies
> cleanly.
> 
> No guarantees, I just remembered this patch and your problem sounds like
> a lost wakeup. Maybe BFQ just drives the sbitmap in a way that triggers the
> symptom.

Yes, symptoms look similar and it happens for devices with shared tagsets
(which megaraid sas is) but that problem usually appeared when there are
lots of LUNs sharing the tagset so that number of tags available per LUN
was rather low. Not sure if that is the case here but probably that patch
is worth a try.

Another thing worth trying is to compile the kernel without
CONFIG_BFQ_GROUP_IOSCHED. That will essentially disable cgroup support in
BFQ so we will see whether the problem may be cgroup related or not.

Another interesting thing might be to dump
/sys/kernel/debug/block/<device>/hctx*/{sched_tags,sched_tags_bitmap,tags,tags_bitmap}
as the system is hanging. That should tell us whether tags are in fact in
use or not when processes are blocking waiting for tags.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR