Re: [PATCH -next v2 1/2] blk-throttle: fix that io throttle can only work for single bio

[Ming Lei <ming.lei@redhat.com>]

On Wed, May 18, 2022 at 03:27:50PM +0800, Yu Kuai wrote:
> commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
> introduce a new problem, for example:
> 
> [root@localhost ~]# echo "8:0 1024" > /sys/fs/cgroup/blkio/blkio.throttle.write_bps_device
> [root@localhost ~]# echo $$ > /sys/fs/cgroup/blkio/cgroup.procs
> [root@localhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
> [1] 620
> [root@localhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
> [2] 626
> [root@localhost ~]# 1+0 records in
> 1+0 records out
> 10240 bytes (10 kB, 10 KiB) copied, 10.0038 s, 1.0 kB/s1+0 records in
> 1+0 records out
> 
> 10240 bytes (10 kB, 10 KiB) copied, 9.23076 s, 1.1 kB/s
> -> the second bio is issued after 10s instead of 20s.
> 
> This is because if some bios are already queued, current bio is queued
> directly and the flag 'BIO_THROTTLED' is set. And later, when former
> bios are dispatched, this bio will be dispatched without waiting at all,
> this is due to tg_with_in_bps_limit() return 0 for this bio.
> 
> In order to fix the problem, don't skip flaged bio in
> tg_with_in_bps_limit(), and for the problem that split bio can be
> double accounted, compensate the over-accounting in __blk_throtl_bio().
> 
> Fixes: 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-throttle.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 447e1b8722f7..6f69859eae23 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 +2118,21 @@ bool __blk_throtl_bio(struct bio *bio)
>  			tg->last_low_overflow_time[rw] = jiffies;
>  		throtl_downgrade_check(tg);
>  		throtl_upgrade_check(tg);
> +
> +		/*
> +		 * re-entered bio has accounted bytes already, so try to
> +		 * compensate previous over-accounting. However, if new
> +		 * slice is started, just forget it.
> +		 */
> +		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;

The above check should be:
			if (tg->last_bytes_disp[rw] >= bio_size)

Otherwise, this patch looks fine for me.


Thanks,
Ming