FIO performance regression in 4.11 kernel vs. 4.10 kernel observed on ARM64

[Scott Branden]

I have updated the kernel to 4.11 and see significant performance
drops using fio-2.9.

Using FIO the performanced drops from 281 KIOPS to 207 KIOPS using
single core and task.
Percent performance drop becomes even worse if multi-cores and multi-
threads are used.

Platform is ARM64 based A72.  Can somebody reproduce the results or
know what may have changed to make such a dramatic change?

FIO command and resulting log output below using null_blk to remove
as many hardware specific driver dependencies as possible.

modprobe null_blk queue_mode=2 irqmode=0 completion_nsec=0
submit_queues=1 bs=4096

taskset 0x1 fio --randrepeat=1 --ioengine=libaio --direct=1 --numjobs=1
--gtod_reduce=1 --name=readtest --filename=/dev/nullb0 --bs=4k
--iodepth=128 --time_based --runtime=15 --readwrite=read

**** 281 KIOPS RESULT on 4.11 Kernel ****
readtest: (g=0): rw=read, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=128
fio-2.9
Starting 1 process
Jobs: 1 (f=1): [R(1)] [100.0% done] [1098MB/0KB/0KB /s] [281K/0/0 iops] 
[eta 00m:00s]
readtest: (groupid=0, jobs=1): err= 0: pid=2868: Mon Apr  3 20:24:25 2017
   read : io=16456MB, bw=1096.1MB/s, iops=280825, runt= 15001msec
   cpu          : usr=28.35%, sys=71.55%, ctx=1560, majf=0, minf=146
   IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, 
 >=64=100.0%
      submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
 >=64=0.0%
      complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
 >=64=0.1%
      issued    : total=r=4212670/w=0/d=0, short=r=0/w=0/d=0, 
drop=r=0/w=0/d=0
      latency   : target=0, window=0, percentile=100.00%, depth=128

Run status group 0 (all jobs):
    READ: io=16456MB, aggrb=1096.1MB/s, minb=1096.1MB/s, 
maxb=1096.1MB/s, mint=15001msec, maxt=15001msec

Disk stats (read/write):
   nullb0: ios=4185627/0, merge=0/0, ticks=3664/0, in_queue=3308, 
util=22.05%


**** 207 KIOPS RESULT on 4.10 Kernel ****
taskset 0x1 fio --randrepeat=1 --ioengine=libaio --direct=1 --numjobs=1 
--gtod_reduce=1 --name=readtest --filename=/dev/nullb0 --bs=4k 
--iodepth=128 --time_based --runtime=15 --readwrite=read
readtest: (g=0): rw=read, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=128
fio-2.9
Starting 1 process
Jobs: 1 (f=1): [R(1)] [100.0% done] [807.6MB/0KB/0KB /s] [207K/0/0 iops] 
[eta 00m:00s]
readtest: (groupid=0, jobs=1): err= 0: pid=2832: Mon Apr  3 20:09:31 2017
   read : io=12109MB, bw=826620KB/s, iops=206654, runt= 15001msec
   cpu          : usr=24.62%, sys=75.28%, ctx=1571, majf=0, minf=146
   IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, 
 >=64=100.0%
      submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
 >=64=0.0%
      complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
 >=64=0.1%
      issued    : total=r=3100030/w=0/d=0, short=r=0/w=0/d=0, 
drop=r=0/w=0/d=0
      latency   : target=0, window=0, percentile=100.00%, depth=128

Run status group 0 (all jobs):
    READ: io=12109MB, aggrb=826619KB/s, minb=826619KB/s, 
maxb=826619KB/s, mint=15001msec, maxt=15001msec

Disk stats (read/write):
   nullb0: ios=3080149/0, merge=0/0, ticks=3952/0, in_queue=3560, 
util=23.73%



Regards,
  Scott