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From:   SeongJae Park <sjpark@amazon.com>
To:     =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
CC:     SeongJae Park <sjpark@amazon.com>, <konrad.wilk@oracle.com>,
        SeongJae Park <sjpark@amazon.de>, <axboe@kernel.dk>,
        <aliguori@amazon.com>, <amit@kernel.org>, <mheyne@amazon.de>,
        <linux-block@vger.kernel.org>, <xen-devel@lists.xenproject.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] xen-blkback: add a parameter for disabling of persistent grants
Date:   Tue, 22 Sep 2020 09:57:32 +0200
Message-ID: <20200922075732.23767-1-sjpark@amazon.com>
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In-Reply-To: <20200922073202.GH19254@Air-de-Roger>
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On Tue, 22 Sep 2020 09:32:02 +0200 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> On Tue, Sep 22, 2020 at 09:01:25AM +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 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@amazon.com>
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >  .../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@amazon.de>
> > +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