netdev - Re: [PATCH net-next v2 4/4] virtio-net: initial debugfs support, export mergeable rx buffer size

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Date:	Sun, 12 Jan 2014 15:32:28 -0800
From:	Michael Dalton <mwdalton@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next v2 4/4] virtio-net: initial debugfs support,
 export mergeable rx buffer size

Hi Michael,

On Sun, Jan 12, 2014 at 9:09 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> Can't we add struct attribute * to netdevice, and pass that in when
> creating the kobj?

I like that idea, I think that will work and should be better than
the alternatives. The actual kobjs for RX queues (struct netdev_rx_queue)
are allocated and deallocated by calls to net_rx_queue_update_kobjects,
which resizes RX queue kobjects when the netdev RX queues are resized.

Is this what you had in mind:
(1) Add a pointer to an attribute group to struct net_device, used for
    per-netdev rx queue attributes and initialized before the call to
    register_netdevice().
(2) Declare an attribute group containing the mergeable_rx_buffer_size
    attribute in virtio-net, and initialize the per-netdevice group pointer
    to the address of this group in virtnet_probe before register_netdevice
(3) In net-sysfs, modify net_rx_queue_update_kobjects
    (or rx_queue_add_kobject) to call sysfs_create_group on the
    per-netdev attribute group (if non-NULL), adding the attributes in
    the group to the RX queue kobject.

That should allow us to have per-RX queue attributes that are
device-specific. I'm not a sysfs expert, but it seems that rx_queue_ktype
and rx_queue_sysfs_ops presume that all rx queue sysfs operations are
performed on attributes of type rx_queue_attribute. That type will need
to be moved from net-sysfs.c to a header file like netdevice.h so that
the type can be used in virtio-net when we declare the
mergeable_rx_buffer_size attribute.

The last issue is how the rx_queue_attribute 'show' function
implementation for mergeable_rx_buffer_size will access the appropriate
per-receive queue EWMA data. The arguments to the show function will be
the netdev_rx_queue and the attribute itself. We can get to the
struct net_device from the netdev_rx_queue.  If we extended
netdev_rx_queue to indicate the queue_index or to store a void *priv_data
pointer, that would be sufficient to allow us to resolve this issue.

Please let me know if the above sounds good or if you see a better way
to accomplish this goal. Thanks!

Best,

Mike
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