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

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Date:   Thu, 13 Dec 2018 14:52:09 -0500
From:   Josef Bacik <josef@...icpanda.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>, 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 Thu, Dec 13, 2018 at 12:48:11PM -0700, Jens Axboe wrote:
> 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.
>

I mean it'll work for now, but then when somebody else wants to do something
similar *cough*io.weight*cough* it'll need a new flag.  I kind of hate adding a
new flag for every controller, but then again it's not like there's thousands of
these things.  I'm having a hard time coming up with a solution other than a
per-tracker flag.  As for this specific version, I still think it needs to be in
iolatency itself, trying to make it generic just means it'll get fucked up again
later down the line.  Thanks,

Josef