Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants

[SeongJae Park <sjpark@amazon.com>]

On Tue, 22 Sep 2020 13:35:30 +0200 "Jürgen Groß" <jgross@suse.com> wrote:

> On 22.09.20 13:26, SeongJae Park wrote:
> > On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> > 
> >> On Tue, Sep 22, 2020 at 12:52:07PM +0200, 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.  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>
> >>> ---
> >>>   .../ABI/testing/sysfs-driver-xen-blkback      |  9 ++++++
> >>>   drivers/block/xen-blkback/xenbus.c            | 28 ++++++++++++++-----
> >>>   2 files changed, 30 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> >>> index ecb7942ff146..ac2947b98950 100644
> >>> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> >>> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> >>> @@ -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).
> >>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> >>> index b9aa5d1ac10b..8a95ddd08b13 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 bool feature_persistent = true;
> >>> +module_param(feature_persistent, bool, 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.
> >>> @@ -906,11 +912,15 @@ 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);
> >>> -	if (err) {
> >>> -		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
> >>> -				 dev->nodename);
> >>> -		goto abort;
> >>> +	if (feature_persistent) {
> >>> +		err = xenbus_printf(xbt, dev->nodename, "feature-persistent",
> >>> +				"%u", feature_persistent);
> >>> +		if (err) {
> >>> +			xenbus_dev_fatal(dev, err,
> >>> +					"writing %s/feature-persistent",
> >>> +					dev->nodename);
> >>> +			goto abort;
> >>> +		}
> >>>   	}
> >>>   
> >>>   	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
> >>> @@ -1093,8 +1103,12 @@ 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);
> >>> +	if (feature_persistent)
> >>> +		pers_grants = xenbus_read_unsigned(dev->otherend,
> >>> +				"feature-persistent", 0);
> >>> +	else
> >>> +		pers_grants = 0;
> >>> +
> >>
> >> Sorry for not realizing earlier, but looking at it again I think you
> >> need to cache the value of feature_persistent when it's first used in
> >> the blkback state data, so that it's consistent.
> >>
> >> What would happen for example with the following flow (assume a
> >> persistent grants enabled frontend):
> >>
> >> feature_persistent = false
> >>
> >> connect(...)
> >> feature-persistent is not written to xenstore
> >>
> >> User changes feature_persistent = true
> >>
> >> connect_ring(...)
> >> pers_grants = true, because feature-persistent is set unconditionally
> >> by the frontend and feature_persistent variable is now true.
> >>
> >> Then blkback will try to use persistent grants and the whole
> >> connection will malfunction because the frontend won't.
> > 
> > Ah, you're right.  I should also catch this before but didn't, sorry.
> > 
> >>
> >> The other option is to prevent changing the variable when there are
> >> blkback instances already running.
> > 
> > I think storing the option value in xenstore would be simpler.  That said, if
> > you prefer this way, please let me know.
> 
> No, Xenstore isn't the right place for that. This is a local
> implementation detail of blkback and shouldn't be exported to Xenstore.

Agreed.  I will try using the 'feature_gnt_persistent' as Roger recommended.


Thanks,
SeongJae Park