On 22.09.20 12:52, SeongJae Park wrote:
From: SeongJae Park <sjpark@xxxxxxxxx>
Persistent grants feature provides high scalability. On some small
systems, however, it could incur data copy overheads[1] and thus it is
required to be disabled. It can be disabled from blkback side using a
module parameter, 'feature_persistent'. But, it is impossible from
blkfront side. For the reason, this commit adds a blkfront module
parameter for disabling of the feature.
[1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability
Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
---
.../ABI/testing/sysfs-driver-xen-blkfront | 9 ++++++
drivers/block/xen-blkfront.c | 28 +++++++++++++------
2 files changed, 29 insertions(+), 8 deletions(-)
diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
index c0a6cb7eb314..9c31334cb2e6 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
@@ -8,3 +8,12 @@ Description:
is 32 - higher value means more potential throughput but more
memory usage. The backend picks the minimum of the frontend
and its default backend value.
+
+What: /sys/module/xen_blkfront/parameters/feature_persistent
+Date: September 2020
+KernelVersion: 5.10
+Contact: SeongJae Park <sjpark@xxxxxxxxx>
+Description:
+ Whether to enable the persistent grants feature or not. Note
+ that this option only takes effect on newly created frontends.
+ The default is Y (enable).
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 91de2e0755ae..49c324f377de 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -149,6 +149,13 @@ static unsigned int xen_blkif_max_ring_order;
module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, 0444);
MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
+/* Enable the persistent grants feature. */
+static bool feature_persistent = true;
+module_param(feature_persistent, bool, 0644);
+MODULE_PARM_DESC(feature_persistent,
+ "Enables the persistent grants feature");
+
+
#define BLK_RING_SIZE(info) \
__CONST_RING_SIZE(blkif, XEN_PAGE_SIZE * (info)->nr_ring_pages)
@@ -1866,11 +1873,13 @@ static int talk_to_blkback(struct xenbus_device *dev,
message = "writing protocol";
goto abort_transaction;
}
- err = xenbus_printf(xbt, dev->nodename,
- "feature-persistent", "%u", 1);
- if (err)
- dev_warn(&dev->dev,
- "writing persistent grants feature to xenbus");
+ if (feature_persistent) {
+ err = xenbus_printf(xbt, dev->nodename,
+ "feature-persistent", "%u", 1);
+ if (err)
+ dev_warn(&dev->dev,
+ "writing persistent grants feature to xenbus");
+ }
err = xenbus_transaction_end(xbt, 0);
if (err) {
@@ -2316,9 +2325,12 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
blkfront_setup_discard(info);
- info->feature_persistent =
- !!xenbus_read_unsigned(info->xbdev->otherend,
- "feature-persistent", 0);
+ if (feature_persistent)
+ info->feature_persistent =
+ !!xenbus_read_unsigned(info->xbdev->otherend,
+ "feature-persistent", 0);
+ else
+ info->feature_persistent = 0;
indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
"feature-max-indirect-segments", 0);
Here you have the same problem as in blkback: feature_persistent could
change its value between the two tests.
Juergen
