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

[yukuai (C)]

在 2022/05/19 17:58, Michal Koutný 写道:
> Hello Kuayi.
>
> On Thu, May 19, 2022 at 04:58:11PM +0800, Yu Kuai <yukuai3@xxxxxxxxxx> 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.
Hi,

The problem is not just zeroes *_disp fields, in fact, the real problem
is that 'slice_start' is reset to jiffies.

> 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?
tg_with_in_bps_limit() is caculating 'wait' based on 'slice_start'and
'bytes_disp':

tg_with_in_bps_limit:
 jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
 tmp = bps_limit * jiffy_elapsed_rnd;
 do_div(tmp, HZ);
 bytes_allowed = tmp; -> how many bytes are allowed in this slice,
		         incluing dispatched.
 if (tg->bytes_disp[rw] + bio_size <= bytes_allowed)
  *wait = 0 -> no need to wait if this bio is within limit

 extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
 -> extra_bytes is based on 'bytes_disp'

For example:

1) bps_limit is 2k, we issue two io, (1k and 9k)
2) the first io(1k) will be dispatched, bytes_disp = 1k, slice_start = 0
   the second io(9k) is waiting for (9 - (2 - 1)) / 2 = 4 s
3) after 3 s, we update bps_limit to 1k, then new waiting is caculated:

without this patch:  bytes_disp = 0, slict_start =3:
bytes_allowed = 1k
extra_bytes = 9k - 1k = 8k
wait = 8s

whth this patch: bytes_disp = 0.5k, slice_start =  0,
bytes_allowed = 1k * 3 + 1k = 4k
extra_bytes =  0.5k + 9k - 4k = 5.5k
wait = 5.5s

I hope I can expliain it clearly...

Thanks,
Kuai
> (Is it because you omit update of slice_{start,end}?)
>
> Thanks,
> Michal
>
> .