linux-kernel - Re: [PATCH v2] block: fix iolat timestamp and restore accounting semantics

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Date:   Thu, 13 Dec 2018 12:48:11 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>
Cc:     kernel-team@...com, linux-block@...r.kernel.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: fix iolat timestamp and restore accounting
 semantics

On 12/11/18 4:01 PM, 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 these issues by reusing the BLK_QUEUE_ENTERED flag to
> determine if a bio has entered the request layer and is responsible for
> starting a request. Stacked drivers don't recurse through
> blk_mq_make_request(), so the overhead of using time between
> generic_make_request() and the blk_mq_get_request() should be minimal.
> blk-iolatency now checks if this flag is set to determine if it should
> process the bio in rq_qos_done_bio().

I'm having a hard time convincing myself that this is correct... Maybe
we should just add a new flag for this specific use case? Or feel free
to convince me otherwise.

-- 
Jens Axboe