[PATCH] block: Consider inflight IO in io accounting for high latency devices

[Gulam Mohamed]

For high latency devices,we need to take into account the inflight IOs
for calculating the disk utilization. For IO accounting in block layer
currently, the io ticks are updated (added) with '1' at the start of the
IO and delta (time difference between start and end of the IO) is added
at the end of the IO. This causes a small issue for high latency
devices.
Multiple IOs can come in before the end of the previous IOs. Suppose,
IO 'A' came in and before its completion a couple of IOs 'B', 'C', 'D'
and 'E' came in. As per the current implementation, we add only 1 to the
ioticks at IO arrival and the disk time 'stamp' is updated with current
time ('now'). Now, just after 'E' IO 'A' completion arrived. For this
completion of 'A', we update the ioticks with delta which is nothing but
"A(end_time) - E(start_time('stamp'))". Eventhough the disk was busy in
processing the IOs B,C,D also, we were missing the processing time of B,C,D
IOs to add to ioticks. This was causing the %util to show the disk
utilization incorrectly. This incorrect behavior was causing it impossible
to drive the disk utilization to 100% even for heavy load.

To fix this, we need to take into account the inflight IOs also for
calculating the disk utilization. While updating the ioticks when IO
arrives, check if there are any inflight IOs. If there are any inflight
IOs, then add delta to the ioticks instead of 1.

This may not be an issue for low latency devices as there will be very
less difference between the start and end of an IO. Also, if we add this
inflight IO check for IO accounting of low latency devices, then it will
be an overhead and impact IO performance.

So, this inflight IO check will be added only to high latency devices
like HDDs and not to low latency devices like NVME. In the fix, this
distinction is made based upon the number of hardware 'queues' supported
by the disk. Usually HDDs support only 1 HW queue and NVME devices support
multiple HW queues.

The following is the fio script used to test the fix with correct results:

[global]
bs=64K
iodepth=120
direct=1
ioengine=libaio
time_based
runtime=100
numjobs=12
name=raw-randread
rw=randread

[job1]
filename=/dev/sda:/dev/sdb:/dev/sdc:/dev/sdd:/dev/sde:/dev/sdf:/dev/sdg:/dev/sdh:/dev/sdi:/dev/sdj:/dev/sdk:/dev/sdm

Results without fix
-------------------
Device   r/s     w/s     rMB/s     wMB/s   rrqm/s   wrqm/s  %rrqm  %wrqm
r_await w_await aqu-sz rareq-sz wareq-sz  svctm  %util
sdj    413.00    0.00    25.81     0.00    0.00     0.00    0.00   0.00
26.75    0.00  11.05    64.00     0.00   1.53   63.30

Result with fix
---------------
Device   r/s     w/s     rMB/s     wMB/s   rrqm/s   wrqm/s  %rrqm  %wrqm
r_await w_await aqu-sz rareq-sz wareq-sz  svctm  %util
sdd      257.00  0.00     16.06      0.00     0.00     0.00   0.00
0.00  101.47    0.00  26.08    64.00     0.00   3.89 100.00

Signed-off-by: Gulam Mohamed <gulam.mohamed@xxxxxxxxxx>
---
 block/blk-core.c | 7 ++++++-
 block/blk-mq.c   | 7 ++++++-
 block/blk.h      | 1 +
 block/genhd.c    | 2 +-
 4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 9d51e9894ece..bc3be34b54fc 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -953,8 +953,13 @@ void update_io_ticks(struct block_device *part, unsigned long now, bool end)
 unsigned long bdev_start_io_acct(struct block_device *bdev, enum req_op op,
 				 unsigned long start_time)
 {
+	bool delta = false;
+
 	part_stat_lock();
-	update_io_ticks(bdev, start_time, false);
+	if (bdev->bd_queue->nr_hw_queues == 1) {
+		delta = !!part_in_flight(bdev);
+	}
+	update_io_ticks(bdev, start_time, delta);
 	part_stat_local_inc(bdev, in_flight[op_is_write(op)]);
 	part_stat_unlock();
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ec922c6bccbe..70e5763fb799 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1000,6 +1000,8 @@ static inline void blk_account_io_done(struct request *req, u64 now)
 
 static inline void blk_account_io_start(struct request *req)
 {
+	bool delta = false;
+
 	trace_block_io_start(req);
 
 	if (blk_do_io_stat(req)) {
@@ -1015,7 +1017,10 @@ static inline void blk_account_io_start(struct request *req)
 			req->part = req->q->disk->part0;
 
 		part_stat_lock();
-		update_io_ticks(req->part, jiffies, false);
+		if (req->q->nr_hw_queues == 1) {
+			delta = !!part_in_flight(req->part);
+		}
+		update_io_ticks(req->part, jiffies, delta);
 		part_stat_unlock();
 	}
 }
diff --git a/block/blk.h b/block/blk.h
index 08a358bc0919..37f778c1c1df 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -292,6 +292,7 @@ ssize_t part_fail_store(struct device *dev, struct device_attribute *attr,
 ssize_t part_timeout_show(struct device *, struct device_attribute *, char *);
 ssize_t part_timeout_store(struct device *, struct device_attribute *,
 				const char *, size_t);
+unsigned int part_in_flight(struct block_device *part);
 
 static inline bool bio_may_exceed_limits(struct bio *bio,
 					 const struct queue_limits *lim)
diff --git a/block/genhd.c b/block/genhd.c
index cc32a0c704eb..8cf16dc7e195 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -118,7 +118,7 @@ static void part_stat_read_all(struct block_device *part,
 	}
 }
 
-static unsigned int part_in_flight(struct block_device *part)
+unsigned int part_in_flight(struct block_device *part)
 {
 	unsigned int inflight = 0;
 	int cpu;
-- 
2.39.3