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@xxxxxxxxxx>
> > Cc: Yu Kuai <yukuai3@xxxxxxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > 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);
>
&carryover_bytes
> + /* carryover_bytes is dispatched without waiting */
> if (!jiffy_wait)
> - 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)
thanks,
Ming
