Hey Tejun and Dennis, I recently found that due to bio_set_dev()'s call to bio_associate_blkg(), bio_set_dev() needs much more cpu than ideal; especially when doing 4K IOs via io_uring's HIPRI bio-polling. I'm very naive about blk-cgroups.. so I'm hopeful you or others can help me cut through this to understand what the ideal outcome should be for DM's bio clone + remap heavy use-case as it relates to bio_associate_blkg. If I hack dm-linear with a local __bio_set_dev that simply removes the call to bio_associate_blkg() my IOPS go from ~980K to 995K. Looking at what is happening a bit, relative to this DM bio cloning usecase, it seems __bio_clone() calls bio_clone_blkg_association() to clone the blkg from DM device, then dm-linear.c:linear_map's call to bio_set_dev() will cause bio_associate_blkg(bio) to reuse the css but then it triggers an update because the bdev is being remapped in the bio (due to linear_map sending the IO to the real underlying device). End result _seems_ like collective wasteful effort to get the blk-cgroup resources setup properly in the face of a simple remap. Seems the current DM pattern is causing repeat blkg work for _every_ remapped bio? Do you see a way to speed up repeat calls to bio_associate_blkg()? Test kernel is my latest dm-5.19 branch (though latest Linus 5.18-rc0 kernel should be fine too): https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-5.19 I'm using dm-linear ontop on a 16G blk-mq null_blk device: modprobe null_blk queue_mode=2 poll_queues=2 bs=4096 gb=16 SIZE=`blockdev --getsz /dev/nullb0` echo "0 $SIZE linear /dev/nullb0 0" | dmsetup create linear And running the workload with fio using this wrapper script: io_uring.sh 20 1 /dev/mapper/linear 4096 #!/bin/bash RTIME=$1 JOBS=$2 DEV=$3 BS=$4 QD=64 BATCH=16 HI=1 fio --bs=$BS --ioengine=io_uring --fixedbufs --registerfiles --hipri=$HI \ --iodepth=$QD \ --iodepth_batch_submit=$BATCH \ --iodepth_batch_complete_min=$BATCH \ --filename=$DEV \ --direct=1 --runtime=$RTIME --numjobs=$JOBS --rw=randread \ --name=test --group_reporting
