Re: [BUG] cgroupv2/blk: inconsistent I/O behavior in Cgroup v2 with set device wbps and wiops

[Yu Kuai]

Hi,

在 2024/08/23 20:05, Lance Yang 写道:
> My bad, I got tied up with some stuff :(
>
> Hmm... tried your debug patch today, but my test results are different from
> yours. So let's take a look at direct IO with raw disk first.
>
> ```
> $ lsblk
> NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
> sda      8:0    0   90G  0 disk
> ├─sda1   8:1    0    1G  0 part /boot/efi
> └─sda2   8:2    0 88.9G  0 part /
> sdb      8:16   0   10G  0 disk
>
> $ cat  /sys/block/sda/queue/scheduler
> none [mq-deadline]
>
> $ cat  /sys/block/sda/queue/rotational
> 0
>
> $ cat  /sys/block/sdb/queue/rotational
> 0
>
> $ cat  /sys/block/sdb/queue/scheduler
> none [mq-deadline]
>
> $ cat /boot/config-6.11.0-rc3+ |grep CONFIG_CGROUP_
> # CONFIG_CGROUP_FAVOR_DYNMODS is not set
> CONFIG_CGROUP_WRITEBACK=y
> CONFIG_CGROUP_SCHED=y
> CONFIG_CGROUP_PIDS=y
> CONFIG_CGROUP_RDMA=y
> CONFIG_CGROUP_FREEZER=y
> CONFIG_CGROUP_HUGETLB=y
> CONFIG_CGROUP_DEVICE=y
> CONFIG_CGROUP_CPUACCT=y
> CONFIG_CGROUP_PERF=y
> CONFIG_CGROUP_BPF=y
> CONFIG_CGROUP_MISC=y
> # CONFIG_CGROUP_DEBUG is not set
> CONFIG_CGROUP_NET_PRIO=y
> CONFIG_CGROUP_NET_CLASSID=y
>
> $ cd /sys/fs/cgroup/test/ && cat cgroup.controllers
> cpu io memory pids
>
> $ cat io.weight
> default 100
>
> $ cat io.prio.class
> no-change
> ```
>
> With wiops, the result is as follows:
>
> ```
> $ echo "8:16 wbps=10485760 wiops=100000" > io.max
>
> $ dd if=/dev/zero of=/dev/sdb bs=50M count=1 oflag=direct
> 1+0 records in
> 1+0 records out
> 52428800 bytes (52 MB, 50 MiB) copied, 5.05893 s, 10.4 MB/s
>
> $ dmesg -T
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 2984 ffff0000fb3a8f00
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 6176 ffff0000fb3a97c0
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 7224 ffff0000fb3a9180
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a8640
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a9400
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a8c80
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a9040
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a92c0
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 4096 ffff0000fb3a8000

> And without wiops, the result is quite different:
>
> ```
> $ echo "8:16 wbps=10485760 wiops=max" > io.max
>
> $ dd if=/dev/zero of=/dev/sdb bs=50M count=1 oflag=direct
> 1+0 records in
> 1+0 records out
> 52428800 bytes (52 MB, 50 MiB) copied, 5.08187 s, 10.3 MB/s
>
> $ dmesg -T
> [Fri Aug 23 10:59:10 2024] __blk_throtl_bio: bio start 2880 ffff0000c74659c0
> [Fri Aug 23 10:59:10 2024] __blk_throtl_bio: bio start 6992 ffff00014f621b80
> [Fri Aug 23 10:59:10 2024] __blk_throtl_bio: bio start 92528 ffff00014f620dc0

I don't know why IO size from fs layer is different in this case.

> ```
>
> Then, I retested for ext4 as you did.
>
> ```
> $ lsblk
> NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
> sda      8:0    0   90G  0 disk
> ├─sda1   8:1    0    1G  0 part /boot/efi
> └─sda2   8:2    0 88.9G  0 part /
> sdb      8:16   0   10G  0 disk
>
> $ df -T /data
> Filesystem     Type 1K-blocks     Used Available Use% Mounted on
> /dev/sda2      ext4  91222760 54648704  31894224  64% /
> ```
>
> With wiops, the result is as follows:
>
> ```
> $ echo "8:0 wbps=10485760 wiops=100000" > io.max
>
> $ rm -rf /data/file1 && dd if=/dev/zero of=/data/file1 bs=50M count=1 oflag=direct
> 1+0 records in
> 1+0 records out
> 52428800 bytes (52 MB, 50 MiB) copied, 5.06227 s, 10.4 MB/s
>
> $ dmesg -T
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 2984 ffff0000fb3a8f00
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 6176 ffff0000fb3a97c0
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 7224 ffff0000fb3a9180
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a8640
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a9400
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a8c80
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a9040
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 16384 ffff0000fb3a92c0
> [Fri Aug 23 11:04:08 2024] __blk_throtl_bio: bio start 4096 ffff0000fb3a8000

> And without wiops, the result is also quite different:
>
> ```
> $ echo "8:0 wbps=10485760 wiops=max" > io.max
>
> $ rm -rf /data/file1 && dd if=/dev/zero of=/data/file1 bs=50M count=1 oflag=direct
> 1+0 records in
> 1+0 records out
> 52428800 bytes (52 MB, 50 MiB) copied, 5.03759 s, 10.4 MB/s
>
> $ dmesg -T
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 2904 ffff0000c4e9f2c0
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 5984 ffff0000c4e9e000
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 7496 ffff0000c4e9e3c0
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 16384 ffff0000c4e9eb40
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 16384 ffff0000c4e9f540
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 16384 ffff0000c4e9e780
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 16384 ffff0000c4e9ea00
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 16384 ffff0000c4e9f900
> [Fri Aug 23 11:05:07 2024] __blk_throtl_bio: bio start 4096 ffff0000c4e9e8c0

While ext4 is the same. And I won't say result is different here.
> [
> ```
>
> Hmm... I still hava two questions here:
> 1. Is wbps an average value?

Yes.
> 2. What's the difference between setting 'max' and setting a very high value for 'wiops'?

The only difference is that:

- If there is no iops limit, splited IO will be dispatched directly;
- If there is iops limit, splited IO will be throttled again. iops is
high, however, blk-throtl is FIFO, splited IO will have to wait for
formal request to be throttled by bps first before checking the iops
limit for splited IO.

Thanks,
Kuai

> Thanks a lot again for your time!
> Lance
> .