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block-throttle: avoid double charge

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If a bio is throttled and split after throttling, the bio could be
resubmited and enters the throttling again. This will cause part of the
bio to be 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/split, we copy the flag to new bio too to avoid a
double charge. However, cloned bio could be directed to a new disk,
keeping the flag be a 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 for a long time, arbitrary bio size change just 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: Vivek Goyal <[email protected]>
Cc: [email protected]
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Shaohua Li <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Shaohua Li 2017-12-20 11:10:17 -07:00 • committed by Jens Axboe
commit 111be88398
4 changed files with 9 additions and 12 deletions
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2
block/bio.c
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@ -599,6 +599,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
bio->bi_disk = bio_src->bi_disk;
bio->bi_partno = bio_src->bi_partno;
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;

8
block/blk-throttle.c
View file

@ -2226,13 +2226,7 @@ again:
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)