[PATCH 1/2] block/blk-throttle: Fix throttle slice time for SSDs

[Guenter Roeck <linux@roeck-us.net>]

Commit d61fcfa4bb18 ("blk-throttle: choose a small throtl_slice for SSD")
introduced device type specific throttle slices if BLK_DEV_THROTTLING_LOW
was enabled. Commit bf20ab538c81 ("blk-throttle: remove
CONFIG_BLK_DEV_THROTTLING_LOW") removed support for BLK_DEV_THROTTLING_LOW,
but left the device type specific throttle slices in place. This
effectively changed throttling behavior on systems with SSD which now use
a different and non-configurable slice time compared to non-SSD devices.
Practical impact is that throughput tests with low configured throttle
values (65536 bps) experience less than expected throughput on SSDs,
presumably due to rounding errors associated with the small throttle slice
time used for those devices. The same tests pass when setting the throttle
values to 65536 * 4 = 262144 bps.

The original code sets the throttle slice time to DFL_THROTL_SLICE_HD if
CONFIG_BLK_DEV_THROTTLING_LOW is disabled. Restore that code to fix the
problem. With that, DFL_THROTL_SLICE_SSD is no longer necessary. Revert to
the original code and re-introduce DFL_THROTL_SLICE to replace both
DFL_THROTL_SLICE_HD and DFL_THROTL_SLICE_SSD. This effectively reverts
commit d61fcfa4bb18 ("blk-throttle: choose a small throtl_slice for SSD").

After the removal of CONFIG_BLK_DEV_THROTTLING_LOW, it is no longer
necessary to enable block accounting, so remove the call to
blk_stat_enable_accounting(). With that, the track_bio_latency variable
is no longer used and can be deleted from struct throtl_data. Also,
including blk-stat.h is no longer necessary.

While at it, also remove MAX_THROTL_SLICE since it is not used anymore.

Fixes: bf20ab538c81 ("blk-throttle: remove CONFIG_BLK_DEV_THROTTLING_LOW")
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 block/blk-throttle.c | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 397b6a410f9e..924d09b51b69 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -12,7 +12,6 @@
 #include <linux/blktrace_api.h>
 #include "blk.h"
 #include "blk-cgroup-rwstat.h"
-#include "blk-stat.h"
 #include "blk-throttle.h"
 
 /* Max dispatch from a group in 1 round */
@@ -22,9 +21,7 @@
 #define THROTL_QUANTUM 32
 
 /* Throttling is performed over a slice and after that slice is renewed */
-#define DFL_THROTL_SLICE_HD (HZ / 10)
-#define DFL_THROTL_SLICE_SSD (HZ / 50)
-#define MAX_THROTL_SLICE (HZ)
+#define DFL_THROTL_SLICE (HZ / 10)
 
 /* A workqueue to queue throttle related work */
 static struct workqueue_struct *kthrotld_workqueue;
@@ -45,8 +42,6 @@ struct throtl_data
 
 	/* Work for dispatching throttled bios */
 	struct work_struct dispatch_work;
-
-	bool track_bio_latency;
 };
 
 static void throtl_pending_timer_fn(struct timer_list *t);
@@ -1345,13 +1340,7 @@ static int blk_throtl_init(struct gendisk *disk)
 		goto out;
 	}
 
-	if (blk_queue_nonrot(q))
-		td->throtl_slice = DFL_THROTL_SLICE_SSD;
-	else
-		td->throtl_slice = DFL_THROTL_SLICE_HD;
-	td->track_bio_latency = !queue_is_mq(q);
-	if (!td->track_bio_latency)
-		blk_stat_enable_accounting(q);
+	td->throtl_slice = DFL_THROTL_SLICE;
 
 out:
 	blk_mq_unquiesce_queue(disk->queue);
-- 
2.45.2