linux-kernel - Re: [PATCH -next v5 4/8] blk-throttle: fix io hung due to config updates

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Date:   Thu, 23 Jun 2022 18:26:20 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, 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
Subject: Re: [PATCH -next v5 4/8] blk-throttle: fix io hung due to config
 updates

On Thu, Jun 23, 2022 at 08:27:11PM +0800, Yu Kuai <yukuai3@...wei.com> wrote:
> > Here we may allow to dispatch a bio above current slice's
> > calculate_bytes_allowed() if bytes_skipped is already >0.
> 
> Hi, I don't expect that to happen. For example, if a bio is still
> throttled, then old slice is keeped with proper 'bytes_skipped',
> then new wait time is caculated based on (bio_size - bytes_skipped).
> 
> After the bio is dispatched(I assum that other bios can't preempt),

With this assumptions it adds up as you write. I believe we're in
agreement.

It's the same assumption I made below (FIFO everywhere, i.e. no
reordering). So the discussed difference shouldn't really be negative
(and if the assumption didn't hold, so the modular arithmetic yields
corerct bytes_skipped value).

Michal