On Wed, Dec 20, 2017 at 02:42:20PM +0800, xuejiufei wrote:

Hi Shaohua,

I noticed that the splitted bio will goto the scheduler directly while
the cloned bio entering the generic_make_request again. So can we just
leave the BIO_THROTTLED flag in the original bio and do not copy the
flag to new splitted bio, so it is not necessary to remove the flag in
bio_set_dev()? Or there are other different situations?

but we can still send the original bio to a different bdev, for example stacked
disks. That bit will prevent throttling for underlayer disks.

Thanks,
Shaohua
 

On 2017/11/14 上午4:37, Shaohua Li wrote:

quoted

If a bio is throttled and splitted after throttling, the bio could be
resubmited and enters the throttling again. This will cause part of the
bio is charged multiple times. If the cgroup has an IO limit, the double
charge will significantly harm the performance. The bio split becomes
quite common after arbitrary bio size change.

To fix this, we always set the BIO_THROTTLED flag if a bio is throttled.
If the bio is cloned/slitted, we copy the flag to new bio too to avoid
double charge. However cloned bio could be directed to a new disk,
keeping the flag will have problem. The observation is we always set new
disk for the bio in this case, so we can clear the flag in
bio_set_dev().

This issue exists a long time, arbitrary bio size change makes it worse,
so this should go into stable at least since v4.2.

V1-> V2: Not add extra field in bio based on discussion with Tejun

Cc: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Shaohua Li <redacted>
---
 block/bio.c          | 2 ++
 block/blk-throttle.c | 8 +-------
 include/linux/bio.h  | 2 ++
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 8338304..d1d4d51 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -598,6 +598,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
 	 */
 	bio->bi_disk = bio_src->bi_disk;
 	bio_set_flag(bio, BIO_CLONED);
+	if (bio_flagged(bio_src, BIO_THROTTLED))
+		bio_set_flag(bio, BIO_THROTTLED);
 	bio->bi_opf = bio_src->bi_opf;
 	bio->bi_write_hint = bio_src->bi_write_hint;
 	bio->bi_iter = bio_src->bi_iter;

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index ee6d7b0..f90fec1 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c

@@ -2222,13 +2222,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 out_unlock:
 	spin_unlock_irq(q->queue_lock);
 out:
-	/*
-	 * As multiple blk-throtls may stack in the same issue path, we
-	 * don't want bios to leave with the flag set.  Clear the flag if
-	 * being issued.
-	 */
-	if (!throttled)
-		bio_clear_flag(bio, BIO_THROTTLED);
+	bio_set_flag(bio, BIO_THROTTLED);
 
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
 	if (throttled || !td->track_bio_latency)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 9c75f58..27b5bac 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h

@@ -504,6 +504,8 @@ extern unsigned int bvec_nr_vecs(unsigned short idx);
 
 #define bio_set_dev(bio, bdev) 			\
 do {						\
+	if ((bio)->bi_disk != (bdev)->bd_disk)	\
+		bio_clear_flag(bio, BIO_THROTTLED);\
 	(bio)->bi_disk = (bdev)->bd_disk;	\
 	(bio)->bi_partno = (bdev)->bd_partno;	\
 } while (0)