On Fri, Apr 11, 2025 at 10:53:12AM +0800, Yu Kuai wrote:
> Hi, Ming
>
> 在 2025/03/05 12:31, Ming Lei 写道:
> > Now ->carryover_bytes[] and ->carryover_ios[] only covers limit/config
> > update.
> >
> > Actually the carryover bytes/ios can be carried to ->bytes_disp[] and
> > ->io_disp[] directly, since the carryover is one-shot thing and only valid
> > in current slice.
> >
> > Then we can remove the two fields and simplify code much.
> >
> > Type of ->bytes_disp[] and ->io_disp[] has to change as signed because the
> > two fields may become negative when updating limits or config, but both are
> > big enough for holding bytes/ios dispatched in single slice
> >
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
> > Cc: Yu Kuai <yukuai3@xxxxxxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > block/blk-throttle.c | 49 +++++++++++++++++++-------------------------
> > block/blk-throttle.h | 4 ++--
> > 2 files changed, 23 insertions(+), 30 deletions(-)
> >
> > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > index 7271aee94faf..91dab43c65ab 100644
> > --- a/block/blk-throttle.c
> > +++ b/block/blk-throttle.c
> > @@ -478,8 +478,6 @@ static inline void throtl_start_new_slice_with_credit(struct throtl_grp *tg,
> > {
> > tg->bytes_disp[rw] = 0;
> > tg->io_disp[rw] = 0;
> > - tg->carryover_bytes[rw] = 0;
> > - tg->carryover_ios[rw] = 0;
> > /*
> > * Previous slice has expired. We must have trimmed it after last
> > @@ -498,16 +496,14 @@ static inline void throtl_start_new_slice_with_credit(struct throtl_grp *tg,
> > }
> > static inline void throtl_start_new_slice(struct throtl_grp *tg, bool rw,
> > - bool clear_carryover)
> > + bool clear)
> > {
> > - tg->bytes_disp[rw] = 0;
> > - tg->io_disp[rw] = 0;
> > + if (clear) {
> > + tg->bytes_disp[rw] = 0;
> > + tg->io_disp[rw] = 0;
> > + }
> > tg->slice_start[rw] = jiffies;
> > tg->slice_end[rw] = jiffies + tg->td->throtl_slice;
> > - if (clear_carryover) {
> > - tg->carryover_bytes[rw] = 0;
> > - tg->carryover_ios[rw] = 0;
> > - }
> > throtl_log(&tg->service_queue,
> > "[%c] new slice start=%lu end=%lu jiffies=%lu",
> > @@ -617,20 +613,16 @@ static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
> > */
> > time_elapsed -= tg->td->throtl_slice;
> > bytes_trim = calculate_bytes_allowed(tg_bps_limit(tg, rw),
> > - time_elapsed) +
> > - tg->carryover_bytes[rw];
> > - io_trim = calculate_io_allowed(tg_iops_limit(tg, rw), time_elapsed) +
> > - tg->carryover_ios[rw];
> > + time_elapsed);
> > + io_trim = calculate_io_allowed(tg_iops_limit(tg, rw), time_elapsed);
> > if (bytes_trim <= 0 && io_trim <= 0)
> > return;
> > - tg->carryover_bytes[rw] = 0;
> > if ((long long)tg->bytes_disp[rw] >= bytes_trim)
> > tg->bytes_disp[rw] -= bytes_trim;
> > else
> > tg->bytes_disp[rw] = 0;
> > - tg->carryover_ios[rw] = 0;
> > if ((int)tg->io_disp[rw] >= io_trim)
> > tg->io_disp[rw] -= io_trim;
> > else
> > @@ -645,7 +637,8 @@ static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
> > jiffies);
> > }
> > -static void __tg_update_carryover(struct throtl_grp *tg, bool rw)
> > +static void __tg_update_carryover(struct throtl_grp *tg, bool rw,
> > + long long *bytes, int *ios)
> > {
> > unsigned long jiffy_elapsed = jiffies - tg->slice_start[rw];
> > u64 bps_limit = tg_bps_limit(tg, rw);
> > @@ -658,26 +651,28 @@ static void __tg_update_carryover(struct throtl_grp *tg, bool rw)
> > * configuration.
> > */
> > if (bps_limit != U64_MAX)
> > - tg->carryover_bytes[rw] +=
> > - calculate_bytes_allowed(bps_limit, jiffy_elapsed) -
> > + *bytes = calculate_bytes_allowed(bps_limit, jiffy_elapsed) -
> > tg->bytes_disp[rw];
> > if (iops_limit != UINT_MAX)
> > - tg->carryover_ios[rw] +=
> > - calculate_io_allowed(iops_limit, jiffy_elapsed) -
> > + *ios = calculate_io_allowed(iops_limit, jiffy_elapsed) -
> > tg->io_disp[rw];
> > + tg->bytes_disp[rw] -= *bytes;
> > + tg->io_disp[rw] -= *ios;
>
> This patch is applied before I get a chance to review. :( I think the
> above update should be:
oops, your review period takes too long(> 1 month), :-(
>
> tg->bytes_disp[rw] = -*bytes;
> tg->io_disp[rw] = -*ios;
I think the above is wrong since it simply override the existed dispatched
bytes/ios.
The calculation can be understood from two ways:
1) delta = calculate_bytes_allowed(bps_limit, jiffy_elapsed) - tg->bytes_disp[rw];
`delta` represents difference between theoretical and actual dispatch bytes.
If `delta` > 0, it means we dispatch too less in past, and we have to subtract
`delta` from ->bytes_disp, so that in future we can dispatch more.
Similar with 'delta < 0'.
2) from consumer viewpoint:
tg_within_bps_limit(): patched
...
bytes_allowed = calculate_bytes_allowed(bps_limit, jiffy_elapsed_rnd);
if (bytes_allowed > 0 && tg->bytes_disp[rw] + bio_size <= bytes_allowed)
...
tg_within_bps_limit(): before patched
...
bytes_allowed = calculate_bytes_allowed(bps_limit, jiffy_elapsed_rnd) +
tg->carryover_bytes[rw];
if (bytes_allowed > 0 && tg->bytes_disp[rw] + bio_size <= bytes_allowed)
...
So if `delta` is subtracted from `bytes_allowed` in patched code, we should
subtract same bytes from ->byte_disp[] side for maintaining the relation.
>
> Otherwise, the result is actually (2 * disp - allowed), which might be a
> huge value, causing long dealy for the next dispatch.
>
> This is what the old carryover fileds do, above change will work, but
> look wried.
As I explained, the patched code follows the original carryover calculation, and it
passes all throt blktests.
Or do you have test case broken by this patch?
Thanks.
Ming
