linux-kernel - Re: [PATCH v3] block: fix blk-iolatency accounting underflow

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Date:   Mon, 17 Dec 2018 09:09:01 -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 v3] block: fix blk-iolatency accounting underflow

On 12/17/18 9:03 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 [1], to tag all bios w/ blkgs undermined how iolatency
> was determining which bios it was charging and should process in
> rq_qos_done_bio(). Because all bios are being tagged, this caused the
> atomic_t for the struct rq_wait inflight count to underflow and result
> in a stall.
> 
> This patch adds a new flag BIO_TRACKED to let controllers know that a
> bio is going through the rq_qos path. blk-iolatency now checks if this
> flag is set to see if it should process the bio in rq_qos_done_bio().
> 
> Overloading BLK_QUEUE_ENTERED works, but makes the flag rules confusing.
> BIO_THROTTLED was another candidate, but the flag is set for all bios
> that have gone through blk-throttle code. Overloading a flag comes with
> the burden of making sure that when either implementation changes, a
> change in setting rules for one doesn't cause a bug in the other. So
> here, we unfortunately opt for adding a new flag.

I think this is better than (ab)using QUEUE_ENTERED, but this is an area
that needs some love and cleanup in the future.

-- 
Jens Axboe