On Fri, Feb 21, 2025 at 10:55:23AM +0800, Ming Lei wrote:
> 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;
Not sure ->carryover_bytes[] can be used here, the comment said
clearly it is only for updating config.
Neither it is good to add one extra, nor add one less, maybe
DIV64_U64_ROUND_CLOSEST() is better?
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8d149aff9fd0..5791612b3543 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -727,16 +727,16 @@ 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_ROUND_CLOSEST(extra_bytes * HZ, bps_limit);
/*
* 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;
+
return jiffy_wait;
}
Thanks,
Ming
