diff mbox series

[v2,2/3] xen-blkfront: add a parameter for disabling of persistent grants

Message ID 20200922105209.5284-3-sjpark@amazon.com (mailing list archive)
State New, archived
Headers show
Series xen-blk(back|front): Let users disable persistent grants | expand

Commit Message

SeongJae Park Sept. 22, 2020, 10:52 a.m. UTC 
From: SeongJae Park <sjpark@amazon.de>

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@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkfront     |  9 ++++++
 drivers/block/xen-blkfront.c                  | 28 +++++++++++++------
 2 files changed, 29 insertions(+), 8 deletions(-)

Comments

Jürgen Groß Sept. 22, 2020, 12:11 p.m. UTC | #1 
On 22.09.20 12:52, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> 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@amazon.de>
> ---
>   .../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@amazon.de>
> +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
SeongJae Park Sept. 22, 2020, 12:44 p.m. UTC | #2 
On Tue, 22 Sep 2020 14:11:32 +0200 "Jürgen Groß" <jgross@suse.com> wrote:

> On 22.09.20 12:52, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> > 
> > 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@amazon.de>
> > ---
> >   .../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->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.

Yes, indeed.  I will fix this in the next version.


Thanks,
SeongJae Park
diff mbox series

Patch

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@amazon.de>
+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);