From: Yu Kuai <yukuai3@huawei.com> stable inclusion from stable-v6.14.2 commit 1c5ceca353089c7948a1064f220aa21e4f904096 category: bugfix bugzilla: https://atomgit.com/openeuler/kernel/issues/9104 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=... -------------------------------- [ Upstream commit 29cb955934302a5da525db6b327c795572538426 ] The bio submission time may be a few jiffies more than the expected waiting time, due to 'extra_bytes' can't be divided in tg_within_bps_limit(), and also due to timer wakeup delay. In this case, adjust slice_start to jiffies will discard the extra wait time, causing lower rate than expected. Current in-tree code already covers deviation by rounddown(), but turns out it is not enough, because jiffies - slice_start can be a multiple of throtl_slice. For example, assume bps_limit is 1000bytes, 1 jiffes is 10ms, and slice is 20ms(2 jiffies), expected rate is 1000 / 1000 * 20 = 20 bytes per slice. If user issues two 21 bytes IO, then wait time will be 30ms for the first IO: bytes_allowed = 20, extra_bytes = 1; jiffy_wait = 1 + 2 = 3 jiffies and consider extra 1 jiffies by timer, throtl_trim_slice() will be called at: jiffies = 40ms slice_start = 0ms, slice_end= 40ms bytes_disp = 21 In this case, before the patch, real rate in the first two slices is 10.5 bytes per slice, and slice will be updated to: jiffies = 40ms slice_start = 40ms, slice_end = 60ms, bytes_disp = 0; Hence the second IO will have to wait another 30ms; With the patch, the real rate in the first slice is 20 bytes per slice, which is the same as expected, and slice will be updated: jiffies=40ms, slice_start = 20ms, slice_end = 60ms, bytes_disp = 1; And now, there is still 19 bytes allowed in the second slice, and the second IO will only have to wait 10ms; This problem will cause blktests throtl/001 failure in case of CONFIG_HZ_100=y, fix it by preserving one extra finished slice in throtl_trim_slice(). Fixes: e43473b7f223 ("blkio: Core implementation of throttle policy") Reported-by: Ming Lei <ming.lei@redhat.com> Closes: https://lore.kernel.org/linux-block/20250222092823.210318-3-yukuai1@huaweicl... Reviewed-by: Ming Lei <ming.lei@redhat.com> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Yu Kuai <yukuai3@huawei.com> Link: https://lore.kernel.org/r/20250227120645.812815-1-yukuai1@huaweicloud.com Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Zizhi Wo <wozizhi@huawei.com> --- block/blk-throttle.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/block/blk-throttle.c b/block/blk-throttle.c index 883255d8f8e0..087a31e50678 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -784,18 +784,27 @@ static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw) * that initially cgroup limit was very low resulting in high * slice_end, but later limit was bumped up and bio was dispatched * sooner, then we need to reduce slice_end. A high bogus slice_end * is bad because it does not allow new slice to start. */ - throtl_set_slice_end(tg, rw, jiffies + tg->td->throtl_slice); time_elapsed = rounddown(jiffies - tg->slice_start[rw], tg->td->throtl_slice); - if (!time_elapsed) + /* Don't trim slice until at least 2 slices are used */ + if (time_elapsed < tg->td->throtl_slice * 2) return; + /* + * The bio submission time may be a few jiffies more than the expected + * waiting time, due to 'extra_bytes' can't be divided in + * tg_within_bps_limit(), and also due to timer wakeup delay. In this + * case, adjust slice_start will discard the extra wait time, causing + * lower rate than expected. Therefore, other than the above rounddown, + * one extra slice is preserved for deviation. + */ + 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]; -- 2.52.0