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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Sep 22, 2020 at 01:26:38PM +0200, 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.

If possible I prefer to avoid reading from xenstore because it's slow,
very slow compared to storing this somewhere in the blkback
structure.

Could you use the feature_gnt_persistent field in xen_vbd maybe as a
place to cache the value before it's set with the final negotiated
value between the frontend and the backend?

Thanks, Roger.