Inject I/O latency for RAID5/6 read and writes

I needed to inject I/O completion latency in the RAID5/6 codes for
test purpose. I was wondering where would be the good place in
md/raid5.c code to add delay ?

So far I have tried adding mdelay/udelay in raid5_end_write_request()
and it seems it works to increase I/O completion of writes. However,
adding delay in raid5_end_read_request() does not really change the
read I/O latency. Any idea ?

--Alireza