Hi Ming,

I'll have a look.

BTW, are you intentionally using scsi_debug over null_blk? null_blk 
supports shared sbitmap as well, and performance figures there are 
generally higher than scsi_debug for similar fio settings.

Thanks,
john

EOM

quoted

		IOPs
mq-deadline	usr=21.72%, sys=44.18%,		423K
none		usr=23.15%, sys=74.01%		450K

Today I re-run the scsi_debug test on two server hardwares(32cores, dual
numa nodes), and the CPU utilization issue can be reproduced, follow
the test result:

1) randread test on ibm-x3850x6[*] with deadline

               |IOPS    | FIO CPU util
------------------------------------------------
hosttags      | 94k    | usr=1.13%, sys=14.75%
------------------------------------------------
non hosttags  | 124k   | usr=1.12%, sys=10.65%,


2) randread test on ibm-x3850x6[*] with none
               |IOPS    | FIO CPU util
------------------------------------------------
hosttags      | 120k   | usr=0.89%, sys=6.55%
------------------------------------------------
non hosttags  | 121k   | usr=1.07%, sys=7.35%
------------------------------------------------

  *:
  	- that is the machine Yanhui reported VM cpu utilization increased by 20%
	- kernel: latest linus tree(v5.12-rc7, commit: 7f75285ca57)
	- also run same test on another 32cores machine, IOPS drop isn't
	  observed, but CPU utilization is increased obviously

3) test script
#/bin/bash

run_fio() {
	RTIME=$1
	JOBS=$2
	DEVS=$3
	BS=$4

	QD=64
	BATCH=16

	fio --bs=$BS --ioengine=libaio \
		--iodepth=$QD \
	    --iodepth_batch_submit=$BATCH \
		--iodepth_batch_complete_min=$BATCH \
		--filename=$DEVS \
		--direct=1 --runtime=$RTIME --numjobs=$JOBS --rw=randread \
		--name=test --group_reporting
}

SCHED=$1

NRQS=`getconf _NPROCESSORS_ONLN`

rmmod scsi_debug
modprobe scsi_debug host_max_queue=128 submit_queues=$NRQS virtual_gb=256
sleep 2
DEV=`lsscsi | grep scsi_debug | awk '{print $6}'`
echo $SCHED >/sys/block/`basename $DEV`/queue/scheduler
echo 128 >/sys/block/`basename $DEV`/device/queue_depth
run_fio 20 16 $DEV 8K


rmmod scsi_debug
modprobe scsi_debug max_queue=128 submit_queues=1 virtual_gb=256
sleep 2
DEV=`lsscsi | grep scsi_debug | awk '{print $6}'`
echo $SCHED >/sys/block/`basename $DEV`/queue/scheduler
echo 128 >/sys/block/`basename $DEV`/device/queue_depth
run_fio 20 16 $DEV 8k