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

Michal Koutný
2022-05-19T09:58:58Z

Hello Kuayi.

On Thu, May 19, 2022 at 04:58:11PM +0800, Yu Kuai <yukuai3@huawei.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