linux-kernel - Re: [PATCH RESEND v6 4/8] blk-throttle: fix io hung due to config updates

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Date:   Thu, 28 Jul 2022 06:55:33 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        axboe@...nel.dk, ming.lei@...hat.com, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RESEND v6 4/8] blk-throttle: fix io hung due to config
 updates

On Thu, Jul 28, 2022 at 06:34:44PM +0800, Yu Kuai wrote:
> Hi
> 
> 在 2022/07/28 17:33, Michal Koutný 写道:
> > On Wed, Jul 27, 2022 at 08:39:19AM -1000, Tejun Heo <tj@...nel.org> wrote:
> > > I'm not quiet sure this is correct. What if the limit keeps changing across
> > > different values? Then we'd be calculating the skipped amount based on the
> > > last configuration only which would be incorrect.
> > 
> > When one change of configuration is correct, then all changes must be
> > correct by induction. It's sufficient to take into account only the one
> > old config and the new one.
> > 
> > This __tg_update_skipped() calculates bytes_skipped with the limit
> > before the change and bytes_skipped are used (divided by) the new limit
> > in tg_with_in_bps_limit().
> > The accumulation of bytes_skipped across multiple changes (until slice
> > properly ends) is proportional to how bytes_allowed would grow over
> > time.
> > That's why I find this correct (I admit I had to look back into my
> > notes when this was first discussed).
> > 
> > HTH,
> > Michal
> > 
> 
> Hi, Tejun
> 
> Michal already explain it very well, please let me know if you still
> thinks there are better ways.

Ah, I see, so it's integrating into the skipped counters across multiple
updates. I think it can definitely use comments explaining how it's working
but that looks okay.

Thanks.

-- 
tejun