[RFC PATCH] dm: fix excessive dm-mq context switching

[Mike Snitzer]

On Sun, Feb 07 2016 at 11:43am -0500,
Sagi Grimberg <sagig at dev.mellanox.co.il> wrote:

> 
> >Hello Sagi,
> 
> Hey Bart,
> 
> >Did you run your test on a NUMA system ?
> 
> I did.
> 
> >If so, can you check with e.g.
> >perf record -ags -e LLC-load-misses sleep 10 && perf report whether this
> >workload triggers perhaps lock contention ? What you need to look for in
> >the perf output is whether any functions occupy more than 10% CPU time.
> 
> I will, thanks for the tip!

Also, I found ftrace's function_graph tracer very helpful (it is how I
found the various issues fixed by the first context switch patch).  Here
is my latest script:

#!/bin/sh

set -xv

NULL_BLK_HW_QUEUES=4
NULL_BLK_QUEUE_DEPTH=4096

DM_MQ_HW_QUEUES=4
DM_MQ_QUEUE_DEPTH=2048

FIO=/root/snitm/git/fio/fio
FIO_QUEUE_DEPTH=32
FIO_RUNTIME=10
FIO_NUMJOBS=12

PERF=perf
#PERF=/root/snitm/git/linux/tools/perf/perf

run_fio() {
    DEVICE=$1
    TASK_NAME=$(basename ${DEVICE})
    PERF_RECORD=$2
    RUN_CMD="${FIO} --cpus_allowed_policy=split --group_reporting --rw=randread --bs=4k --numjobs=${FIO_NUMJOBS} \
              --iodepth=${FIO_QUEUE_DEPTH} --runtime=${FIO_RUNTIME} --time_based --loops=1 --ioengine=libaio \
              --direct=1 --invalidate=1 --randrepeat=1 --norandommap --exitall --name task_${TASK_NAME} --filename=${DEVICE}"

    if [ ! -z "${PERF_RECORD}" ]; then
    ${PERF_RECORD} ${RUN_CMD}
    mv perf.data perf.data.${TASK_NAME}
    else
    ${RUN_CMD}
    fi
}

run_fio_with_ftrace() {
    DEVICE=$1
    TASK_NAME=$(basename ${DEVICE})

    echo > /sys/kernel/debug/tracing/trace
    echo 0 > /sys/kernel/debug/tracing/tracing_on
    echo function_graph > /sys/kernel/debug/tracing/current_tracer
    echo 1 > /sys/kernel/debug/tracing/tracing_on
    run_fio $DEVICE
    echo 0 > /sys/kernel/debug/tracing/tracing_on
    cat /sys/kernel/debug/tracing/trace > trace.${TASK_NAME}
    echo nop > /sys/kernel/debug/tracing/current_tracer
}

dmsetup remove dm_mq
modprobe -r null_blk

modprobe null_blk gb=4 bs=512 hw_queue_depth=${NULL_BLK_QUEUE_DEPTH} nr_devices=1 queue_mode=2 irqmode=1 completion_nsec=1 submit_queues=${NULL_BLK_HW_QUEUES}
#run_fio /dev/nullb0 "${PERF} record -ag -e cs"
#run_fio /dev/nullb0 "${PERF} stat"

echo Y > /sys/module/dm_mod/parameters/use_blk_mq
echo ${DM_MQ_QUEUE_DEPTH} > /sys/module/dm_mod/parameters/blk_mq_queue_depth
echo ${DM_MQ_HW_QUEUES} > /sys/module/dm_mod/parameters/blk_mq_nr_hw_queues
echo "0 8388608 multipath 0 0 1 1 service-time 0 1 2 /dev/nullb0 1000 1" | dmsetup create dm_mq
#echo "0 8388608 linear /dev/nullb0 0" | dmsetup create dm_mq

run_fio_with_ftrace /dev/mapper/dm_mq

#run_fio /dev/mapper/dm_mq
#run_fio /dev/mapper/dm_mq "${PERF} record -ag -e cs"
#run_fio /dev/mapper/dm_mq "${PERF} record -ag"
#run_fio /dev/mapper/dm_mq "${PERF} stat"

#run_fio /dev/mapper/dm_mq "trace-cmd record -e all"