[PATCH 2/2] blkback: Add a module parameter for aggressive pool shrinking duration

[SeongJae Park <sjpark@amazon.com>]

From: SeongJae Park <sjpark@amazon.de>

As discussed by the previous commit ("xen/blkback: Aggressively shrink
page pools if a memory pressure is detected"), the aggressive pool
shrinking duration should be carefully selected:
``If it is too long, free pages pool shrinking overhead can reduce the
I/O performance.  If it is too short, blkback will not free enough pages
to reduce the memory pressure.``

That said, the proper duration would depends on given configurations and
workloads.  For the reason, this commit allows users to set it via a
module parameter interface.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Suggested-by: Amit Shah <aams@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index aa1a127093e5..88c011300ee9 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -137,9 +137,13 @@ module_param(log_stats, int, 0644);
 
 /*
  * Once a memory pressure is detected, keep aggressive shrinking of the free
- * page pools for this time (msec)
+ * page pools for this time (milliseconds)
  */
-#define AGGRESSIVE_SHRINKING_DURATION	1
+static int xen_blkif_aggressive_shrinking_duration = 1;
+module_param_named(aggressive_shrinking_duration,
+		xen_blkif_aggressive_shrinking_duration, int, 0644);
+MODULE_PARM_DESC(aggressive_shrinking_duration,
+"Duration to do aggressive shrinking when a memory pressure is detected");
 
 static unsigned long xen_blk_mem_pressure_end;
 
@@ -147,7 +151,7 @@ static unsigned long blkif_shrink_count(struct shrinker *shrinker,
 				struct shrink_control *sc)
 {
 	xen_blk_mem_pressure_end = jiffies +
-		msecs_to_jiffies(AGGRESSIVE_SHRINKING_DURATION);
+		msecs_to_jiffies(xen_blkif_aggressive_shrinking_duration);
 	return 0;
 }
 
-- 
2.17.1