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Subject: Re: [PATCH] block: fix iolat timestamp and restore accounting
 semantics
To:     Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Josef Bacik <josef@toxicpanda.com>
Cc:     kernel-team@fb.com, linux-block@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20181210163510.58985-1-dennis@kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <f698e396-2577-b0d9-e300-f65d4f062986@kernel.dk>
Date:   Mon, 10 Dec 2018 09:58:13 -0700
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On 12/10/18 9:35 AM, Dennis Zhou wrote:
> The blk-iolatency controller measures the time from rq_qos_throttle() to
> rq_qos_done_bio() and attributes this time to the first bio that needs
> to create the request. This means if a bio is plug-mergeable or
> bio-mergeable, it gets to bypass the blk-iolatency controller.
> 
> The recent series, to tag all bios w/ blkgs in [1] changed the timing
> incorrectly as well. First, the iolatency controller was tagging bios
> and using that information if it should process it in rq_qos_done_bio().
> However, now that all bios are tagged, this caused the atomic_t for the
> struct rq_wait inflight count to underflow resulting in a stall. Second,
> now the timing was using the duration a bio from generic_make_request()
> rather than the timing mentioned above.
> 
> This patch fixes the errors by accounting time separately in a bio
> adding the field bi_start. If this field is set, the bio should be
> processed by blk-iolatency in rq_qos_done_bio().
> 
> [1] https://lore.kernel.org/lkml/20181205171039.73066-1-dennis@kernel.org/

Looks reasonable to me, but it needs a Fixes tag as well.

-- 
Jens Axboe