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From:   Liu Bo <obuil.liubo@gmail.com>
Date:   Mon, 17 Dec 2018 15:23:38 -0800
Message-ID: <CANQeFDCeRk0L2iHu=nTGXV8baP2bUc+JRba3TY6QkpKaQcY+qw@mail.gmail.com>
Subject: Re: [PATCH v3] block: fix blk-iolatency accounting underflow
To:     Dennis Zhou <dennis@kernel.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>, kernel-team@fb.com,
        linux-block@vger.kernel.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Mon, Dec 17, 2018 at 1:28 PM Dennis Zhou <dennis@kernel.org> wrote:
>
> On Mon, Dec 17, 2018 at 11:42:28AM -0800, Liu Bo wrote:
> > On Mon, Dec 17, 2018 at 8:04 AM Dennis Zhou <dennis@kernel.org> 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.
> > >
> >
> > Hi,
> >
> > I have a question about merging in plug list, since plug merges are
> > done before rq_qos_throttle(), why would plug-mergeable bios bypass
> > the controller?
> >
> > thanks,
> > liubo
> >
>
> Hi Liubo,
>
> BIO_TRACKED is tagging the bio that is responsible for allocating a new
> request, so that rq_qos controllers can decide whether or not they want
> to process the bio any part of the way. I should have phrased that a
> little better in the commit message. It's not that the bio itself is
> bypassing the blk-iolatency controller, but the blk-iolatency controller
> deciding to not do anything based on the BIO_TRACKED flag. This doesn't
> change any of the function calls made on a bio/request.
>

Thanks for the explanation.
I see it now, so the mentioned series had associated all bios with a
blkg if possible so that done_bio() bumps up inflight counter even in
case a bio has been merged in a previous request.

BIO_TRACKED seems to be too generic to use, but otherwise it makes sense to me,
Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>

thanks,
liubo

> Thanks,
> Dennis