io_uring/sqpoll: statistics of the true utilization of sq threads

io_uring/sqpoll: statistics of the true utilization of sq threads

Count the running time and actual IO processing time of the sqpoll
thread, and output the statistical data to fdinfo.

Variable description:
"work_time" in the code represents the sum of the jiffies of the sq
thread actually processing IO, that is, how many milliseconds it
actually takes to process IO. "total_time" represents the total time
that the sq thread has elapsed from the beginning of the loop to the
current time point, that is, how many milliseconds it has spent in
total.

The test tool is fio, and its parameters are as follows:
[global]
ioengine=io_uring
direct=1
group_reporting
bs=128k
norandommap=1
randrepeat=0
refill_buffers
ramp_time=30s
time_based
runtime=1m
clocksource=clock_gettime
overwrite=1
log_avg_msec=1000
numjobs=1

[disk0]
filename=/dev/nvme0n1
rw=read
iodepth=16
hipri
sqthread_poll=1

The test results are as follows:
Every 2.0s: cat /proc/9230/fdinfo/6 | grep -E Sq
SqMask: 0x3
SqHead: 3197153
SqTail: 3197153
CachedSqHead:   3197153
SqThread:       9231
SqThreadCpu:    11
SqTotalTime:    18099614
SqWorkTime:     16748316

The test results corresponding to different iodepths are as follows:
|-----------|-------|-------|-------|------|-------|
|   iodepth |   1   |   4   |   8   |  16  |  64   |
|-----------|-------|-------|-------|------|-------|
|utilization| 2.9%  | 8.8%  | 10.9% | 92.9%| 84.4% |
|-----------|-------|-------|-------|------|-------|
|    idle   | 97.1% | 91.2% | 89.1% | 7.1% | 15.6% |
|-----------|-------|-------|-------|------|-------|

Signed-off-by: Xiaobing Li <xiaobing.li@samsung.com>
Link: https://lore.kernel.org/r/20240228091251.543383-1-xiaobing.li@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>