Hello Kuayi.

On Thu, May 19, 2022 at 04:58:11PM +0800, Yu Kuai [off-list ref] 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