Re: [PATCH v2 2/3] xen-blkfront: add a parameter for disabling of persistent grants
On Tue, 22 Sep 2020 14:11:32 +0200 "Jürgen Groß" wrote:
> On 22.09.20 12:52, SeongJae Park wrote:
> > From: SeongJae Park
> >
> > 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
> > ---
> > .../ABI/testing/sysfs-driver-xen-blkfront | 9 ++
> > drivers/block/xen-blkfront.c | 28 +--
> > 2 files changed, 29 insertions(+), 8 deletions(-)
> >
[...]
> > 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,
> > -"writing persistent grants feature to xenbus");
> > + if (feature_persistent) {
> > + err = xenbus_printf(xbt, dev->nodename,
> > + "feature-persistent", "%u", 1);
> > + if (err)
> > + dev_warn(>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.
Yes, indeed. I will fix this in the next version.
Thanks,
SeongJae Park
Re: [PATCH v2 2/3] xen-blkfront: add a parameter for disabling of persistent grants
On 22.09.20 12:52, SeongJae Park wrote:
From: SeongJae Park
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
---
.../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
+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,
-"writing persistent grants feature to xenbus");
+ if (feature_persistent) {
+ err = xenbus_printf(xbt, dev->nodename,
+ "feature-persistent", "%u", 1);
+ if (err)
+ dev_warn(>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
