linux-kernel - Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

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Message-ID: <20180118212327.GB31679@redhat.com>
Date:   Thu, 18 Jan 2018 16:23:27 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "osandov@...com" <osandov@...com>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>
Subject: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

On Thu, Jan 18 2018 at  3:58P -0500,
Bart Van Assche <Bart.VanAssche@....com> wrote:

> On Thu, 2018-01-18 at 15:48 -0500, Mike Snitzer wrote:
> > For Bart's test the underlying scsi-mq driver is what is regularly
> > hitting this case in __blk_mq_try_issue_directly():
> > 
> >         if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))
> 
> Hello Mike,
> 
> That code path is not the code path that triggered the lockups that I reported
> during the past days.

If you're hitting blk_mq_sched_insert_request() then you most certainly
are hitting that code path.

If you aren't then what was your earlier email going on about?
https://www.redhat.com/archives/dm-devel/2018-January/msg00372.html

If you were just focusing on that as one possible reason, that isn't
very helpful.  By this point you really should _know_ what is triggering
the stall based on the code paths taken.  Please use ftrace's
function_graph tracer if need be.

> These lockups were all triggered by incorrect handling of
> .queue_rq() returning BLK_STS_RESOURCE.

Please be precise, dm_mq_queue_rq()'s return of BLK_STS_RESOURCE?
"Incorrect" because it no longer runs blk_mq_delay_run_hw_queue()?

Please try to do more work analyzing the test case that only you can
easily run (due to srp_test being a PITA).  And less time lobbying for
a change that you don't understand to _really_ be correct.

We have time to get this right, please stop hyperventilating about
"regressions".

Thanks,
Mike