Re: [PATCH] block: throttle: don't add one extra jiffy mistakenly for bps limit

[Ming Lei <ming.lei@redhat.com>]

On Fri, Feb 21, 2025 at 11:39:17AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/02/21 10:55, Ming Lei 写道:
> > Hi Yukuai,
> > 
> > On Thu, Feb 20, 2025 at 09:38:12PM +0800, Yu Kuai wrote:
> > > Hi,
> > > 
> > > 在 2025/02/20 19:17, Ming Lei 写道:
> > > > When the current bio needs to be throttled because of bps limit, the wait
> > > > time for the extra bytes may be less than 1 jiffy, tg_within_bps_limit()
> > > > adds one extra 1 jiffy.
> > > > 
> > > > However, when taking roundup time into account, the extra 1 jiffy
> > > > may become not necessary, then bps limit becomes not accurate. This way
> > > > causes blktests throtl/001 failure in case of CONFIG_HZ_100=y.
> > > > 
> > > > Fix it by not adding the 1 jiffy in case that the roundup time can
> > > > cover it.
> > > > 
> > > > Cc: Tejun Heo <tj@kernel.org>
> > > > Cc: Yu Kuai <yukuai3@huawei.com>
> > > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > > ---
> > > >    block/blk-throttle.c | 6 +++---
> > > >    1 file changed, 3 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > > > index 8d149aff9fd0..8348972c517b 100644
> > > > --- a/block/blk-throttle.c
> > > > +++ b/block/blk-throttle.c
> > > > @@ -729,14 +729,14 @@ static unsigned long tg_within_bps_limit(struct throtl_grp *tg, struct bio *bio,
> > > >    	extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
> > > >    	jiffy_wait = div64_u64(extra_bytes * HZ, bps_limit);
> > > > -	if (!jiffy_wait)
> > > > -		jiffy_wait = 1;
> > > > -
> > > >    	/*
> > > >    	 * This wait time is without taking into consideration the rounding
> > > >    	 * up we did. Add that time also.
> > > >    	 */
> > > >    	jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
> > > > +	if (!jiffy_wait)
> > > > +		jiffy_wait = 1;
> > > 
> > > Just wonder, will wait (0, 1) less jiffies is better than wait (0, 1)
> > > more jiffies.
> > > 
> > > How about following changes?
> > > 
> > > Thanks,
> > > Kuai
> > > 
> > > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > > index 8d149aff9fd0..f8430baf3544 100644
> > > --- a/block/blk-throttle.c
> > > +++ b/block/blk-throttle.c
> > > @@ -703,6 +703,7 @@ static unsigned long tg_within_bps_limit(struct
> > > throtl_grp *tg, struct bio *bio,
> > >                                  u64 bps_limit)
> > >   {
> > >          bool rw = bio_data_dir(bio);
> > > +       long long carryover_bytes;
> > >          long long bytes_allowed;
> > >          u64 extra_bytes;
> > >          unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
> > > @@ -727,10 +728,11 @@ static unsigned long tg_within_bps_limit(struct
> > > throtl_grp *tg, struct bio *bio,
> > > 
> > >          /* Calc approx time to dispatch */
> > >          extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
> > > -       jiffy_wait = div64_u64(extra_bytes * HZ, bps_limit);
> > > +       jiffy_wait = div64_u64_rem(extra_bytes * HZ, bps_limit,
> > > carryover_bytes);
> Hi, Thanks for the test.
> 
> This is a mistake, carryover_bytes is much bigger than expected :(
> That's why the result is much worse. My bad.
> > > 
> > 
> > &carryover_bytes
> > 
> > > +       /* carryover_bytes is dispatched without waiting */
> > >          if (!jiffy_wait)
> The if condition shound be removed.
> > > -               jiffy_wait = 1;
> > > +               tg->carryover_bytes[rw] -= carryover_bytes;
> > > 
> > >          /*
> > >           * This wait time is without taking into consideration the rounding
> > > 
> > > > +
> > > >    	return jiffy_wait;
> > 
> > Looks result is worse with your patch:
> > 
> > throtl/001 (basic functionality)                             [failed]
> >      runtime  6.488s  ...  28.862s
> >      --- tests/throtl/001.out	2024-11-21 09:20:47.514353642 +0000
> >      +++ /root/git/blktests/results/nodev/throtl/001.out.bad	2025-02-21 02:51:36.723754146 +0000
> >      @@ -1,6 +1,6 @@
> >       Running throtl/001
> >      +13
> >       1
> >      -1
> >      -1
> >      +13
> >       1
> >      ...
> >      (Run 'diff -u tests/throtl/001.out /root/git/blktests/results/nodev/throtl/001.out.bad' to see the entire diff)
> 
> And I realize now that throtl_start_new_slice() will just clear
> the carryover_bytes, I tested in my VM and with following changes,
> throtl/001 never fail with CONFIG_HZ_100.

If carryover_bytes can cover this issue, I think it is preferred.

> 
> Thanks,
> Kuai
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 8d149aff9fd0..4fc005af82e0 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -703,6 +703,7 @@ static unsigned long tg_within_bps_limit(struct
> throtl_grp *tg, struct bio *bio,
>                                 u64 bps_limit)
>  {
>         bool rw = bio_data_dir(bio);
> +       long long carryover_bytes;
>         long long bytes_allowed;
>         u64 extra_bytes;
>         unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
> @@ -727,10 +728,8 @@ static unsigned long tg_within_bps_limit(struct
> throtl_grp *tg, struct bio *bio,
> 
>         /* Calc approx time to dispatch */
>         extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
> -       jiffy_wait = div64_u64(extra_bytes * HZ, bps_limit);
> -
> -       if (!jiffy_wait)
> -               jiffy_wait = 1;
> +       jiffy_wait = div64_u64_rem(extra_bytes * HZ, bps_limit,
> &carryover_bytes);
> +       tg->carryover_bytes[rw] -= div64_u64(carryover_bytes, HZ);

Can you explain a bit why `carryover_bytes/HZ` is subtracted instead of
carryover_bytes?

Also tg_within_bps_limit() may return 0 now, which isn't expected.


Thanks, 
Ming