blk-throttle: fix that io throttle can only work for single bio

Test scripts:
cd /sys/fs/cgroup/blkio/
echo "8:0 1024" > blkio.throttle.write_bps_device
echo $$ > cgroup.procs
dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &

Test result:
10240 bytes (10 kB, 10 KiB) copied, 10.0134 s, 1.0 kB/s
10240 bytes (10 kB, 10 KiB) copied, 10.0135 s, 1.0 kB/s

The problem is that the second bio is finished after 10s instead of 20s.

Root cause:
1) second bio will be flagged:

__blk_throtl_bio
 while (true) {
  ...
  if (sq->nr_queued[rw]) -> some bio is throttled already
   break
 };
 bio_set_flag(bio, BIO_THROTTLED); -> flag the bio

2) flagged bio will be dispatched without waiting:

throtl_dispatch_tg
 tg_may_dispatch
  tg_with_in_bps_limit
   if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED))
    *wait = 0; -> wait time is zero
    return true;

commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
support to count split bios for iops limit, thus it adds flagged bio
checking in tg_with_in_bps_limit() so that split bios will only count
once for bps limit, however, it introduce a new problem that io throttle
won't work if multiple bios are throttled.

In order to fix the problem, handle iops/bps limit in different ways:

1) for iops limit, there is no flag to record if the bio is throttled,
   and iops is always applied.
2) for bps limit, original bio will be flagged with BIO_BPS_THROTTLED,
   and io throttle will ignore bio with the flag.

Noted this patch also remove the code to set flag in __bio_clone(), it's
introduced in commit 111be8839817 ("block-throttle: avoid double
charge"), and author thinks split bio can be resubmited and throttled
again, which is wrong because split bio will continue to dispatch from
caller.

Fixes: 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
Cc: <stable@vger.kernel.org>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220829022240.3348319-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 6 insertions, 14 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 667b2958471a..a5e495b67827 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -811,7 +811,7 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
 	unsigned int bio_size = throtl_bio_data_size(bio);
 
 	/* no need to throttle if this bio's bytes have been accounted */
-	if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED)) {
+	if (bps_limit == U64_MAX || bio_flagged(bio, BIO_BPS_THROTTLED)) {
 		if (wait)
 			*wait = 0;
 		return true;
@@ -921,22 +921,13 @@ static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
 	unsigned int bio_size = throtl_bio_data_size(bio);
 
 	/* Charge the bio to the group */
-	if (!bio_flagged(bio, BIO_THROTTLED)) {
+	if (!bio_flagged(bio, BIO_BPS_THROTTLED)) {
 		tg->bytes_disp[rw] += bio_size;
 		tg->last_bytes_disp[rw] += bio_size;
 	}
 
 	tg->io_disp[rw]++;
 	tg->last_io_disp[rw]++;
-
-	/*
-	 * BIO_THROTTLED is used to prevent the same bio to be throttled
-	 * more than once as a throttled bio will go through blk-throtl the
-	 * second time when it eventually gets issued.  Set it when a bio
-	 * is being charged to a tg.
-	 */
-	if (!bio_flagged(bio, BIO_THROTTLED))
-		bio_set_flag(bio, BIO_THROTTLED);
 }
 
 /**
@@ -1026,6 +1017,7 @@ 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
@@ -2181,8 +2173,10 @@ again:
 		qn = &tg->qnode_on_parent[rw];
 		sq = sq->parent_sq;
 		tg = sq_to_tg(sq);
-		if (!tg)
+		if (!tg) {
+			bio_set_flag(bio, BIO_BPS_THROTTLED);
 			goto out_unlock;
+		}
 	}
 
 	/* out-of-limit, queue to @tg */
@@ -2211,8 +2205,6 @@ again:
 	}
 
 out_unlock:
-	bio_set_flag(bio, BIO_THROTTLED);
-
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
 	if (throttled || !td->track_bio_latency)
 		bio->bi_issue.value |= BIO_ISSUE_THROTL_SKIP_LATENCY;