On Tue, 22 Sep 2020 09:32:02 +0200 "Roger Pau Monné" <roger.pau@xxxxxxxxxx> wrote: > On Tue, Sep 22, 2020 at 09:01:25AM +0200, SeongJae Park wrote: > > From: SeongJae Park <sjpark@xxxxxxxxx> > > > > Persistent grants feature provides high scalability. On some small > > systems, however, it could incur data copy overhead[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. > > Have you considered adding a similar option for blkfront? I will add yet another option for blkfront in the next spin. > > > > > [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability > > > > Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx> > > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> > > --- > > .../ABI/testing/sysfs-driver-xen-blkback | 8 ++++++++ > > drivers/block/xen-blkback/xenbus.c | 17 ++++++++++++++--- > > 2 files changed, 22 insertions(+), 3 deletions(-) > > > > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback > > index ecb7942ff146..0c42285c75ee 100644 > > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback > > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback > > @@ -35,3 +35,11 @@ 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@xxxxxxxxx> > > +Description: > > + Whether to enable the persistent grants feature or not. > > + The default is 1 (enable). > > I would add that this option only takes effect on newly created > backends, existing backends when the option is set will continue to > use persistent grants. > > For already running backends you could drain the buffer of persistent > grants and flip the option, but that's more complex and not required. You're right, I will add the description. > > > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c > > index b9aa5d1ac10b..9c03d70469f4 100644 > > --- a/drivers/block/xen-blkback/xenbus.c > > +++ b/drivers/block/xen-blkback/xenbus.c > > @@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev) > > > > /* ** Connection ** */ > > > > +/* Enable the persistent grants feature. */ > > +static unsigned int feature_persistent = 1; > > +module_param_named(feature_persistent, feature_persistent, int, 0644); > > +MODULE_PARM_DESC(feature_persistent, > > + "Enables the persistent grants feature"); > > + > > /* > > * Write the physical details regarding the block device to the store, and > > * switch to Connected state. > > @@ -906,7 +912,8 @@ static void connect(struct backend_info *be) > > > > 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", > > + feature_persistent ? 1 : 0); > > You can avoid writing the feature altogether if it's not enabled, > there's no need to set feature-persistent = 0. Agreed. I will do so in the next spin. Thanks, SeongJae Park
