linux-kernel - [PATCH v3 2/9] blk-throttle: Fix that bps of child could exceed bps limited in parent

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Message-Id: <20221205115709.251489-3-shikemeng@huaweicloud.com>
Date:   Mon,  5 Dec 2022 19:57:02 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk
Cc:     cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, shikemeng@...wei.com,
        linfeilong@...wei.com, liuzhiqiang26@...wei.com
Subject: [PATCH v3 2/9] blk-throttle: Fix that bps of child could exceed bps limited in parent

From: Kemeng Shi <shikemeng@...wei.com>

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.

Signed-off-by: Kemeng Shi <shikemeng@...wei.com>
Acked-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
---
 block/blk-throttle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8e2349b17936..2444ebf5f11d 100644
--- 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);
-- 
2.30.0