blk-throttle: Fix that bps of child could exceed bps limited in parent

Consider situation as following (on the default hierarchy):
 HDD
  |
root (bps limit: 4k)
  |
child (bps limit :8k)
  |
fio bs=8k
Rate of fio is supposed to be 4k, but result is 8k. Reason is as
following:
Size of single IO from fio is larger than bytes allowed in one
throtl_slice in child, so IOs are always queued in child group first.
When queued IOs in child are dispatched to parent group, BIO_BPS_THROTTLED
is set and these IOs will not be limited by tg_within_bps_limit anymore.
Fix this by only set BIO_BPS_THROTTLED when the bio traversed the entire
tree.

There patch has no influence on situation which is not on the default
hierarchy as each group is a single root group without parent.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Link: https://lore.kernel.org/r/20221205115709.251489-3-shikemeng@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8e2349b..2444ebf 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1067,7 +1067,6 @@ static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
        sq->nr_queued[rw]--;
 
        throtl_charge_bio(tg, bio);
-       bio_set_flag(bio, BIO_BPS_THROTTLED);
 
        /*
         * If our parent is another tg, we just need to transfer @bio to
@@ -1080,6 +1079,7 @@ static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
                throtl_add_bio_tg(bio, &tg->qnode_on_parent[rw], parent_tg);
                start_parent_slice_with_credit(tg, parent_tg, rw);
        } else {
+               bio_set_flag(bio, BIO_BPS_THROTTLED);
                throtl_qnode_add_bio(bio, &tg->qnode_on_parent[rw],
                                     &parent_sq->queued[rw]);
                BUG_ON(tg->td->nr_queued[rw] <= 0);