[PATCH v7 1/9] blk-throttle: fix that io throttle can only work for single bio

[Yu Kuai <yukuai1@huaweicloud.com>]

From: Yu Kuai <yukuai3@huawei.com>

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 flaged:

__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) flaged 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 splited bios for iops limit, thus it adds flaged bio
checking in tg_with_in_bps_limit() so that splited 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, at first, don't skip flaged bio in
tg_with_in_bps_limit(), however, this will break that splited bios should
only count once for bps limit. And this patch tries to avoid
over-accounting by decrementing it first in __blk_throtl_bio(), and
then counting it again while dispatching it.

Fixes: 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-throttle.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 9f5fe62afff9..2957e2c643f4 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) {
 		if (wait)
 			*wait = 0;
 		return true;
@@ -921,11 +921,8 @@ 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)) {
-		tg->bytes_disp[rw] += bio_size;
-		tg->last_bytes_disp[rw] += bio_size;
-	}
-
+	tg->bytes_disp[rw] += bio_size;
+	tg->last_bytes_disp[rw] += bio_size;
 	tg->io_disp[rw]++;
 	tg->last_io_disp[rw]++;
 
@@ -2121,6 +2118,23 @@ bool __blk_throtl_bio(struct bio *bio)
 			tg->last_low_overflow_time[rw] = jiffies;
 		throtl_downgrade_check(tg);
 		throtl_upgrade_check(tg);
+
+		/*
+		 * Splited bios can be re-entered because iops limit should be
+		 * counted again, however, bps limit should not. Since bps limit
+		 * will be counted again while dispatching it, compensate the
+		 * over-accounting here. Noted that compensation can fail if
+		 * new slice is started.
+		 */
+		if (bio_flagged(bio, BIO_THROTTLED)) {
+			unsigned int bio_size = throtl_bio_data_size(bio);
+
+			if (tg->bytes_disp[rw] >= bio_size)
+				tg->bytes_disp[rw] -= bio_size;
+			if (tg->last_bytes_disp[rw] >= bio_size)
+				tg->last_bytes_disp[rw] -= bio_size;
+		}
+
 		/* throtl is FIFO - if bios are already queued, should queue */
 		if (sq->nr_queued[rw])
 			break;
-- 
2.31.1