Re: [PATCH -next v4 4/4] blk-throttle: fix io hung due to config updates

[Yu Kuai]

在 2022/05/24 17:59, Michal Koutný 写道:
> On Mon, May 23, 2022 at 04:26:33PM +0800, Yu Kuai <yukuai3@xxxxxxxxxx> wrote:
> > Fix the problem by respecting the time that throttled bio aready waited.
> > In order to do that, add new fields to record how many bytes/io already
> > waited, and use it to calculate wait time for throttled bio under new
> > configuration.
>
> This new approach is correctly conserving the bandwidth upon changes.
> (Looking and BPS paths.)
>
> > Some simple test:
> > 1)
> > cd /sys/fs/cgroup/blkio/
> > echo $$ > cgroup.procs
> > echo "8:0 2048" > blkio.throttle.write_bps_device
> > {
> >          sleep 3
> >          echo "8:0 1024" > blkio.throttle.write_bps_device
> > } &
> > sleep 1
> > dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct
> >
> > 2)
> > cd /sys/fs/cgroup/blkio/
> > echo $$ > cgroup.procs
> > echo "8:0 1024" > blkio.throttle.write_bps_device
> > {
> >          sleep 5
> >          echo "8:0 2048" > blkio.throttle.write_bps_device
> > } &
> > sleep 1
> > dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct
>
> It's interesting that you're getting these numbers (w/patch)
>
> > test results: io finish time
> > 	before this patch	with this patch
> > 1)	10s			6s
> > 2)	8s			6s
>
> wait := (disp + bio - Δt*l_old) / l_new
>
> 1)
> wait = (0k + 8k - 3s*2k/s) / 1k/s = 2s -> i.e. 5s absolute
>
> 2)
> wait = (0k + 8k - 5s*1k/s) / 2k/s = 2.5s -> i.e. 6.5s absolute
>
> Are you numbers noisy+rounded or do I still mis anything?
Hi, Michal

The way of your caculation is right, however, it seems like you missed
that io is dispatched after 1s:

sleep 1  -> here
dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct
> (Also isn't it worth having this more permanent in tools/testing/selftest?)
>
> > +static void tg_update_skipped(struct throtl_grp *tg)
> > +{
> > +	if (tg->service_queue.nr_queued[READ])
> > +		__tg_update_skipped(tg, READ);
> > +	if (tg->service_queue.nr_queued[WRITE])
> > +		__tg_update_skipped(tg, WRITE);
>
> On one hand, the callers of tg_update_skipped() know whether R/W limit
> is changed, so only the respective variant could be called.
> On the other hand, this conditions look implied by tg->flags &
> THROTL_TG_PENDING.
> (Just noting, it's likely still not possibly to pass the skipped value
> only via stack.)
>
>
> > @@ -115,6 +115,10 @@ struct throtl_grp {
> >   	uint64_t bytes_disp[2];
> >   	/* Number of bio's dispatched in current slice */
> >   	unsigned int io_disp[2];
> > +	/* Number of bytes will be skipped in current slice */
> > +	uint64_t bytes_skipped[2];
> > +	/* Number of bio's will be skipped in current slice */
> > +	unsigned int io_skipped[2];
>
> Please add a comment these fields exists to facilitate config updates
> (the bytes to be skipped is sort of obvious from the name :-).
Ok, will do that in next iteration.

Thanks,
Kuai