[dm-devel] question about block-throttle on data device of dm-thin pool

[Vivek Goyal]

On Tue, Jan 10, 2017 at 02:47:02PM +0800, Hou Tao wrote:
> Hi, all.
> 
> I am trying to test block-throttle on dm-thin devices. I find the throttling
> on dm-thin device is OK, but the throttling doesn't work for the data device
> of dm-thin pool.
> 
> The following is my test case:
> #!/bin/sh
> 
> dmsetup create pool --table '0 41943040 thin-pool /dev/vdb /dev/vda \
> 	128 6553 1 skip_block_zeroing
> dmsetup message /dev/mapper/pool 0 'create_thin 1'
> dmsetup create thin_1 --table '0 41943040 thin /dev/mapper/pool 1'
> 
> mp=/thin_1
> mkfs.xfs /dev/mapper/thin_1
> mount /dev/mapper/thin_1 $mp
> 
> cg=/sys/fs/cgroup/blkio/test
> mkdir -p $cg
> # get the block device id of the data device
> data_dev=$(dmsetup table /dev/mapper/pool | awk '{print $5}')
> echo "${data_dev} 1048576" > $cg/blkio.throttle.write_bps_device
> echo $$ > $cg/cgroup.procs
> dd if=/dev/zero of=$mp/zero bs=1M count=1 oflag=direct
> 
> I read the dm-thin code roughly and find out that most bios are submitted
> by the workqueue of thin pool instead of the dd process which initiates the
> O_DIRECT write operations. The bios belong to the block cgroup "blkcg_root"
> instead of the created block cgroup "test" in test case, so the write
> limitation of blkcg "test" doesn't work.
> 
> In order to make the throttling work out, can we save the original block
> cgroup info of the deferred bios and use the saved block cgroup info to
> submit the bios ? Is the method reasonable and is there a better way to
> complete the throttling on the data device of the thin pool ?

I thought we had a patches where bio_clone_bioset() also retained
cgroup information of original bio.

commit 20bd723ec6a3261df5e02250cd3a1fbb09a343f2
Author: Paolo Valente <paolo.valente at linaro.org>
Date:   Wed Jul 27 07:22:05 2016 +0200

    block: add missing group association in bio-cloning functions

Are you running new enough kernel. If not, may be there are still
some places either in generic code or dm code where cloned/newly
created bios are attributed to root cgroup and not to the cgroup
of bio which caused creation of that new bio.

Vivek