[PATCH 1/2] xen/blkback: turn the cache purge LRU interval into a parameter

[Roger Pau Monne <roger.pau@citrix.com>]

Assume that reads and writes to the variable will be atomic. The worse
that could happen is that one of the LRU intervals is not calculated
properly if a partially written value is read, but that would only be
a transient issue.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: xen-devel@lists.xenproject.org
Cc: linux-block@vger.kernel.org
Cc: J. Roeleveld <joost@antarean.org>
Cc: Jürgen Groß <jgross@suse.com>
---
 Documentation/ABI/testing/sysfs-driver-xen-blkback | 10 ++++++++++
 drivers/block/xen-blkback/blkback.c                |  9 ++++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index ecb7942ff146..776f25d335ca 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -35,3 +35,13 @@ Description:
                 controls the duration in milliseconds that blkback will not
                 cache any page not backed by a grant mapping.
                 The default is 10ms.
+
+What:           /sys/module/xen_blkback/parameters/lru_internval
+Date:           October 2020
+KernelVersion:  5.10
+Contact:        Roger Pau Monné <roger.pau@citrix.com>
+Description:
+                The LRU mechanism to clean the lists of persistent grants needs
+                to be executed periodically. This parameter controls the time
+                interval between consecutive executions of the purge mechanism
+                is set in ms.
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index adfc9352351d..6ad9b76fdb2b 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -117,7 +117,10 @@ MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the
  * be executed periodically. The time interval between consecutive executions
  * of the purge mechanism is set in ms.
  */
-#define LRU_INTERVAL 100
+static unsigned int lru_interval = 100;
+module_param_named(lru_interval, lru_interval, uint, 0644);
+MODULE_PARM_DESC(lru_internval,
+		 "Time interval between consecutive executions of the cache purge mechanism (in ms)");
 
 /*
  * When the persistent grants list is full we will remove unused grants
@@ -620,7 +623,7 @@ int xen_blkif_schedule(void *arg)
 		if (unlikely(vbd->size != vbd_sz(vbd)))
 			xen_vbd_resize(blkif);
 
-		timeout = msecs_to_jiffies(LRU_INTERVAL);
+		timeout = msecs_to_jiffies(lru_interval);
 
 		timeout = wait_event_interruptible_timeout(
 			ring->wq,
@@ -650,7 +653,7 @@ int xen_blkif_schedule(void *arg)
 		if (blkif->vbd.feature_gnt_persistent &&
 		    time_after(jiffies, ring->next_lru)) {
 			purge_persistent_gnt(ring);
-			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
+			ring->next_lru = jiffies + msecs_to_jiffies(lru_interval);
 		}
 
 		/* Shrink the free pages pool if it is too large. */
-- 
2.28.0