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Date:   Wed, 23 Sep 2020 16:09:30 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To:     SeongJae Park <sjpark@amazon.com>
Cc:     roger.pau@citrix.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
Message-ID: <20200923200930.GB11767@char.us.oracle.com>
References: <20200922070125.27251-1-sjpark@amazon.com>
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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.

Would it be better suited to have it per guest?
> 
> [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).
> 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);
>  	if (err) {
>  		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
>  				 dev->nodename);
> @@ -1093,8 +1100,12 @@ 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;
> +
>  	blkif->vbd.feature_gnt_persistent = pers_grants;
>  	blkif->vbd.overflow_max_grants = 0;
>  
> -- 
> 2.17.1
>