Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v3] block: fix blk-iolatency accounting underflow
To:     Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.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: <20181217160351.27903-1-dennis@kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <bd74dbe2-0f96-a2aa-043a-6f570c4ffb12@kernel.dk>
Date:   Mon, 17 Dec 2018 09:09:01 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.2.1
MIME-Version: 1.0
In-Reply-To: <20181217160351.27903-1-dennis@kernel.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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