Blk-throttle: reduce tail io latency when iops limit is enforced

When an application's iops has exceeded its cgroup's iops limit, surely it
is throttled and kernel will set a timer for dispatching, thus IO latency
includes the delay.

However, the dispatch delay which is calculated by the limit and the
elapsed jiffies is suboptimal.  As the dispatch delay is only calculated
once the application's iops is (iops limit + 1), it doesn't need to wait
any longer than the remaining time of the current slice.

The difference can be proved by the following fio job and cgroup iops
setting,
-----
$ echo 4 > /mnt/config/nullb/disk1/mbps    # limit nullb's bandwidth to 4MB/s for testing.
$ echo "253:1 riops=100 rbps=max" > /sys/fs/cgroup/unified/cg1/io.max
$ cat r2.job
[global]
name=fio-rand-read
filename=/dev/nullb1
rw=randread
bs=4k
direct=1
numjobs=1
time_based=1
runtime=60
group_reporting=1

[file1]
size=4G
ioengine=libaio
iodepth=1
rate_iops=50000
norandommap=1
thinktime=4ms
-----

wo patch:
file1: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=1
fio-3.7-66-gedfc
Starting 1 process

   read: IOPS=99, BW=400KiB/s (410kB/s)(23.4MiB/60001msec)
    slat (usec): min=10, max=336, avg=27.71, stdev=17.82
    clat (usec): min=2, max=28887, avg=5929.81, stdev=7374.29
     lat (usec): min=24, max=28901, avg=5958.73, stdev=7366.22
    clat percentiles (usec):
     |  1.00th=[    4],  5.00th=[    4], 10.00th=[    4], 20.00th=[    4],
     | 30.00th=[    4], 40.00th=[    4], 50.00th=[    6], 60.00th=[11731],
     | 70.00th=[11863], 80.00th=[11994], 90.00th=[12911], 95.00th=[22676],
     | 99.00th=[23725], 99.50th=[23987], 99.90th=[23987], 99.95th=[25035],
     | 99.99th=[28967]

w/ patch:
file1: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=1
fio-3.7-66-gedfc
Starting 1 process

   read: IOPS=100, BW=400KiB/s (410kB/s)(23.4MiB/60005msec)
    slat (usec): min=10, max=155, avg=23.24, stdev=16.79
    clat (usec): min=2, max=12393, avg=5961.58, stdev=5959.25
     lat (usec): min=23, max=12412, avg=5985.91, stdev=5951.92
    clat percentiles (usec):
     |  1.00th=[    3],  5.00th=[    3], 10.00th=[    4], 20.00th=[    4],
     | 30.00th=[    4], 40.00th=[    5], 50.00th=[   47], 60.00th=[11863],
     | 70.00th=[11994], 80.00th=[11994], 90.00th=[11994], 95.00th=[11994],
     | 99.00th=[11994], 99.50th=[11994], 99.90th=[12125], 99.95th=[12125],
     | 99.99th=[12387]

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>

Signed-off-by: Jens Axboe <axboe@kernel.dk>