linux-kernel - Re: [PATCH -next v3 2/2] blk-throttle: fix io hung due to configuration updates

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Date:   Thu, 19 May 2022 11:58:58 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, ming.lei@...hat.com,
        geert@...ux-m68k.org, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next v3 2/2] blk-throttle: fix io hung due to
 configuration updates

Hello Kuayi.

On Thu, May 19, 2022 at 04:58:11PM +0800, Yu Kuai <yukuai3@...wei.com> wrote:
> If new configuration is submitted while a bio is throttled, then new
> waiting time is recaculated regardless that the bio might aready wait
> for some time:
> 
> tg_conf_updated
>  throtl_start_new_slice
>   tg_update_disptime
>   throtl_schedule_next_dispatch
> 
> Then io hung can be triggered by always submmiting new configuration
> before the throttled bio is dispatched.

O.K.

> -	/*
> -	 * We're already holding queue_lock and know @tg is valid.  Let's
> -	 * apply the new config directly.
> -	 *
> -	 * Restart the slices for both READ and WRITES. It might happen
> -	 * that a group's limit are dropped suddenly and we don't want to
> -	 * account recently dispatched IO with new low rate.
> -	 */
> -	throtl_start_new_slice(tg, READ);
> -	throtl_start_new_slice(tg, WRITE);
> +	throtl_update_slice(tg, old_limits);

throtl_start_new_slice zeroes *_disp fields.
If for instance, new config allowed only 0.5 throughput, the *_disp
fields would be scaled to 0.5.
How that change helps (better) the previously throttled bio to be dispatched?

(Is it because you omit update of slice_{start,end}?)

Thanks,
Michal