bfq: Limit waker detection in time

bfq: Limit waker detection in time

Currently, when process A starts issuing requests shortly after process
B has completed some IO three times in a row, we decide that B is a
"waker" of A meaning that completing IO of B is needed for A to make
progress and generally stop separating A's and B's IO much. This logic
is useful to avoid unnecessary idling and thus throughput loss for cases
where workload needs to switch e.g. between the process and the
journaling thread doing IO. However the detection heuristic tends to
frequently give false positives when A and B are fighting IO bandwidth
and other processes aren't doing much IO as we are basically deemed to
eventually accumulate three occurences of a situation where one process
starts issuing requests after the other has completed some IO. To reduce
these false positives, cancel the waker detection also if we didn't
accumulate three detected wakeups within given timeout. The rationale is
that if wakeups are really rare, the pointless idling doesn't hurt
throughput that much anyway.

This significantly reduces false waker detection for workload like:

[global]
directory=/mnt/repro/
rw=write
size=8g
time_based
runtime=30
ramp_time=10
blocksize=1m
direct=0
ioengine=sync

[slowwriter]
numjobs=1
fsync=200

[fastwriter]
numjobs=1
fsync=200

Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211125133645.27483-5-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>