linux-kernel - Re: [PATCH -next v3 1/2] blk-throttle: fix that io throttle can only work for single bio

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Date:   Thu, 19 May 2022 18:42:41 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, geert@...ux-m68k.org,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH -next v3 1/2] blk-throttle: fix that io throttle can only
 work for single bio

On Thu, May 19, 2022 at 04:58:10PM +0800, Yu Kuai wrote:
> commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
> introduce a new problem, for example:
> 
> [root@...alhost ~]# echo "8:0 1024" > /sys/fs/cgroup/blkio/blkio.throttle.write_bps_device
> [root@...alhost ~]# echo $$ > /sys/fs/cgroup/blkio/cgroup.procs
> [root@...alhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
> [1] 620
> [root@...alhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
> [2] 626
> [root@...alhost ~]# 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@...wei.com>
> ---

Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thanks,
Ming