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>

1 files changed, 2 insertions, 0 deletions

diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 820cb8c2d1fe..bb8180c52a31 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -388,6 +388,8 @@ struct bfq_queue {
 	struct bfq_queue *tentative_waker_bfqq;
 	/* number of times the same tentative waker has been detected */
 	unsigned int num_waker_detections;
+	/* time when we started considering this waker */
+	u64 waker_detection_started;
 
 	/* node for woken_list, see below */
 	struct hlist_node woken_list_node;