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xen-blkback: add a parameter for disabling of persistent grants 

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.  But, there is no option to disable it.  For
the reason, this commit adds a module parameter for disabling of the
feature.

[1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability

Signed-off-by: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200923061841.20531-2-sjpark@amazon.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
This commit is contained in:
SeongJae Park 2020-09-23 08:18:39 +02:00 committed by Boris Ostrovsky
parent 5f7f77400a
commit aac8a70db2
2 changed files with 25 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
Documentation/ABI/testing/sysfs-driver-xen-blkback
View File

@ -35,3 +35,12 @@ 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/feature_persistent
Date: September 2020
KernelVersion: 5.10
Contact: SeongJae Park <sjpark@amazon.de>
Description:
Whether to enable the persistent grants feature or not. Note
that this option only takes effect on newly created backends.
The default is Y (enable).

22
drivers/block/xen-blkback/xenbus.c
View File

@ -473,6 +473,12 @@ static void xen_vbd_free(struct xen_vbd *vbd)
vbd->bdev = NULL;
}
/* 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");
static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
unsigned major, unsigned minor, int readonly,
int cdrom)
@ -518,6 +524,8 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
if (q && blk_queue_secure_erase(q))
vbd->discard_secure = true;
vbd->feature_gnt_persistent = feature_persistent;
pr_debug("Successful creation of handle=%04x (dom=%u)\n",
handle, blkif->domid);
return 0;
@ -905,7 +913,8 @@ again:
xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);
err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
be->blkif->vbd.feature_gnt_persistent);
if (err) {
xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
dev->nodename);
@ -1066,7 +1075,6 @@ static int connect_ring(struct backend_info *be)
{
struct xenbus_device *dev = be->dev;
struct xen_blkif *blkif = be->blkif;
unsigned int pers_grants;
char protocol[64] = "";
int err, i;
char *xspath;
@ -1092,9 +1100,11 @@ static int connect_ring(struct backend_info *be)
xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
return -ENOSYS;
}
pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
0);
blkif->vbd.feature_gnt_persistent = pers_grants;
if (blkif->vbd.feature_gnt_persistent)
blkif->vbd.feature_gnt_persistent =
xenbus_read_unsigned(dev->otherend,
"feature-persistent", 0);
blkif->vbd.overflow_max_grants = 0;
/*
@ -1117,7 +1127,7 @@ static int connect_ring(struct backend_info *be)
pr_info("%s: using %d queues, protocol %d (%s) %s\n", dev->nodename,
blkif->nr_rings, blkif->blk_protocol, protocol,
pers_grants ? "persistent grants" : "");
blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");
ring_page_order = xenbus_read_unsigned(dev->otherend,
"ring-page-order", 0);