linux-kernel - Re: [PATCH v2] blk-iolatency: fix STS

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Date:   Fri, 5 Jul 2019 15:14:44 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Dennis Zhou <dennis@...nel.org>, Josef Bacik <josef@...icpanda.com>
Cc:     kernel-team@...com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blk-iolatency: fix STS_AGAIN handling

On 7/5/19 3:09 PM, Dennis Zhou wrote:
> The iolatency controller is based on rq_qos. It increments on
> rq_qos_throttle() and decrements on either rq_qos_cleanup() or
> rq_qos_done_bio(). a3fb01ba5af0 fixes the double accounting issue where
> blk_mq_make_request() may call both rq_qos_cleanup() and
> rq_qos_done_bio() on REQ_NO_WAIT. So checking STS_AGAIN prevents the
> double decrement.
> 
> The above works upstream as the only way we can get STS_AGAIN is from
> blk_mq_get_request() failing. The STS_AGAIN handling isn't a real
> problem as bio_endio() skipping only happens on reserved tag allocation
> failures which can only be caused by driver bugs and already triggers
> WARN.
> 
> However, the fix creates a not so great dependency on how STS_AGAIN can
> be propagated. Internally, we (Facebook) carry a patch that kills read
> ahead if a cgroup is io congested or a fatal signal is pending. This
> combined with chained bios progagate their bi_status to the parent is
> not already set can can cause the parent bio to not clean up properly
> even though it was successful. This consequently leaks the inflight
> counter and can hang all IOs under that blkg.
> 
> To nip the adverse interaction early, this removes the rq_qos_cleanup()
> callback in iolatency in favor of cleaning up always on the
> rq_qos_done_bio() path.

Looks good, applied for 5.3. Thanks Dennis.

-- 
Jens Axboe