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

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Message-ID: <CAK1f24=AJ81+hqQ+KK4wWceGCcSQWyyrb-ekRXu4_9gDM88PCQ@mail.gmail.com>
Date: Mon, 26 Aug 2024 10:15:38 +0800
From: Lance Yang <ioworker0@...il.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: 21cnbao@...il.com, a.hindborg@...sung.com, axboe@...nel.dk, 
	baolin.wang@...ux.alibaba.com, boqun.feng@...il.com, cgroups@...r.kernel.org, 
	david@...hat.com, fujita.tomonori@....ntt.co.jp, josef@...icpanda.com, 
	libang.li@...group.com, linux-block@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, mkoutny@...e.com, 
	paolo.valente@...more.it, tj@...nel.org, vbabka@...nel.org, 
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [BUG] cgroupv2/blk: inconsistent I/O behavior in Cgroup v2 with
 set device wbps and wiops

Hi Kuai,

Thanks a lot for following up on this!

On Mon, Aug 26, 2024 at 9:31 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> 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.

Me neither...

>
> > ```
> >
> > 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.

Perhap there is other subtle stuff at play since ext4 is the same?

> > [
> > ```
> >
> > 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 a lot again for the lesson!
Lance

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