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 18:34:44 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Michal Koutný <mkoutny@...e.com>,
        Tejun Heo <tj@...nel.org>
Cc:     Yu Kuai <yukuai1@...weicloud.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

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.

Thanks,
Kuai