Re: [PATCH] Add Multiple TX/RX Queues Support for WWAN Network Device

Hello Xiayu,

On Mon, Dec 13, 2021 at 9:06 AM Xiayu Zhang [off-list ref] wrote:

Thanks for your constructive inputs, and sorry for late response.

On Fri, 2021-12-10 at 02:11 +0300, Sergey Ryazanov wrote:

quoted

On Wed, Dec 8, 2021 at 7:04 AM [off-list ref] wrote:

quoted

This patch adds 2 callback functions get_num_tx_queues() and
get_num_rx_queues() to let WWAN network device driver customize its
own
TX and RX queue numbers. It gives WWAN driver a chance to implement
its
own software strategies, such as TX Qos.

Currently, if WWAN device driver creates default bearer interface
when
calling wwan_register_ops(), there will be only 1 TX queue and 1 RX
queue
for the WWAN network device. In this case, driver is not able to
enlarge
the queue numbers by calling netif_set_real_num_tx_queues() or
netif_set_real_num_rx_queues() to take advantage of the network
device's
capability of supporting multiple TX/RX queues.

As for additional interfaces of secondary bearers, if userspace
service
doesn't specify the num_tx_queues or num_rx_queues in netlink
message or
iproute2 command, there also will be only 1 TX queue and 1 RX queue
for
each additional interface. If userspace service specifies the
num_tx_queues
and num_rx_queues, however, these numbers could be not able to
match the
capabilities of network device.

Besides, userspace service is hard to learn every WWAN network
device's
TX/RX queue numbers.

In order to let WWAN driver determine the queue numbers, this patch
adds
below callback functions in wwan_ops:
    struct wwan_ops {
        unsigned int priv_size;
        ...
        unsigned int (*get_num_tx_queues)(unsigned int hint_num);
        unsigned int (*get_num_rx_queues)(unsigned int hint_num);
    };

WWAN subsystem uses the input parameters num_tx_queues and
num_rx_queues of
wwan_rtnl_alloc() as hint values, and passes the 2 values to the
two
callback functions. WWAN device driver should determine the actual
numbers
of network device's TX and RX queues according to the hint value
and
device's capabilities.

As already stated by Jakub, it is hard to understand a new API
suitability without an API user. I will try to describe possible
issues with the proposed API as far as I understand its usage and
possible solutions. Correct me if I am wrong.

There are actually two tasks related to the queues number selection:
1) default queues number selection if the userspace provides no
information about a wishful number of queues;
2) rejecting the new netdev (bearer) creation if a requested number
of queues seems to be invalid.

Your proposal tries to solve both of these tasks with a single hook
that silently increases or decreases the requested number of queues.
This is creative, but seems contradictory to regular RTNL behavior.
RTNL usually selects a correct default value if no value was
requested, or performs what is requested, or explicitly rejects
requested configuration.

You could handle an invalid queues configuration in the .newlink
callback. This callback is even able to return a string error
representation via the extack argument.

As for the default queues number selection it seems better to
implement the RTNL .get_num_rx_queues callback in the WWAN core and
call optional driver specific callback through it. Something like
this:

static unsigned int wwan_rtnl_get_num_tx_queues(struct nlattr *tb[])
{
    const char *devname = nla_data(tb[IFLA_PARENT_DEV_NAME]);
    struct wwan_device *wwandev = wwan_dev_get_by_name(devname);

    return wwandev && wwandev->ops && wwandev->ops->get_num_tx_queues
?
              wwandev->ops->get_num_tx_queues() : 1;
}

static struct rtnl_link_ops wwan_rtnl_link_ops __read_mostly = {
    ...
    .get_num_tx_queues = wwan_rtnl_get_num_tx_queues,
};

This way the default queues number selection will be implemented in a
less surprising way.

But to be able to implement this we need to modify the RTNL ops
.get_num_tx_queues/.get_num_rx_queues callback definitions to make
them able to accept the RTM_NEWLINK message attributes. This is not
difficult since the callbacks are implemented only by a few virtual
devices, but can be assumed too intrusive to implement a one feature
for a single subsystem.

Indeed, I had considered this solution provided by you as well:

   static unsigned int wwan_rtnl_get_num_tx_queues(struct nlattr *tb[])

   static struct rtnl_link_ops wwan_rtnl_link_ops __read_mostly = {
      ...
      .get_num_tx_queues = wwan_rtnl_get_num_tx_queues,
   };

I totally agree that it follows the design of RTNL better.

There are some reasons that let me not apply the solution above, I want
to share them with you. Please correct me if I'm wrong.

   1) in rtnl_create_link, RTNL always prefers to use the number
   provided by userspace service rather than the number returned by
   get_num_tx/rx_queues() of WWAN Core:

      if (tb[IFLA_NUM_TX_QUEUES])
         num_tx_queues = nla_get_u32(tb[IFLA_NUM_TX_QUEUES]);
      else if (ops->get_num_tx_queues)
         num_tx_queues = ops->get_num_tx_queues();

   Although WWAN driver could reject the number selected by userspace
   service in newlink function, this will require userspace service to
   learn this error and implement its retry machanisms. Of course, even
   so, that's not bad.

Why do you assume that a userspace service must provide the
IFLA_NUM_TX_QUEUES attribute?

This attribute is optional, see below.

   I think it's probably better to let WWAN device driver determine
   its default queue number.

Exactly! If we provide RTNL with a .get_num_tx_queues() callback, then
in case of missed IFLA_NUM_TX_QUEUES attribute, RTNL will select the
number of queues according to the driver decision. And only if
userspace forces the driver to use a particular number of queues using
the IFLA_NUM_TX_QUEUES attribute, then RTNL will try to use a
non-default queues number. In that case, the driver may reject the
creation of such a bearer.

So, with the .get_num_tx_queues() callback we will have a simple
scheme. Either, userspace does not specify the IFLA_NUM_TX_QUEUES
attribute and allows the driver to select an appropriate number of
queues. Or, userspace would like to force a specific number of queues
using the IFLA_NUM_TX_QUEUES attribute, but in that case, the
userspace application should be ready to receive a rejection.

   2) As you described, above solution will modify the definition and
   usage of get_num_tx_queues() and get_num_rx_queues() in
   rtnl_link_ops. Userspace service also needs to add new NETLINK
   attributes.

What new attributes did you mean?

IFLA_NUM_TX_QUEUES is optional as shown above. The
IFLA_PARENT_DEV_NAME attribute must be provided anyway, otherwise the
WWAN subsystem will not be able to locate a particular driver and the
interface (bearer) creation request will be rejected. Attributes
already are passed to the WWAN subsystem via the .rtnl_alloc()
callback. I suggest to pass the same attributes to the
.get_num_tx_queues() callback that will be called against the same
RTM_NEWLINK message, just slightly earlier.

   3) WWAN subsystem implements the rtnl_link_ops and plays a role of
   the bridge between RTNL and WWAN device driver. As a separate
   subsystem, I think it could be able to supply its own callback
   functions to WWAN device driver in wwan_ops just as shown in this
   patch.

Yep, we need a callback to be able to support multi-queue modems. I am
just not happy with a callback that silently tries to improve a user's
choice. And I would like to find a more straightforward solution for
multi-queue support.

   In addition to these reasons, I also agree with your points:
      "can be assumed too intrusive to implement a one feature for
      a single subsystem."

But it looks like we have no choice here other than extending the
.get_num_tx_queues() prototype.

Is there any RTNL guru here who could explain whether it is acceptable
to extend the internal API for a single subsystem?

Please review my thoughts and give me some inputs at your convenience.

quoted

quoted

Signed-off-by: Xiayu Zhang <redacted>
---
 drivers/net/wwan/wwan_core.c | 25 ++++++++++++++++++++++++-
 include/linux/wwan.h         |  6 ++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/wwan_core.c

b/drivers/net/wwan/wwan_core.c
index d293ab688044..00095c6987be 100644

--- a/drivers/net/wwan/wwan_core.c
+++ b/drivers/net/wwan/wwan_core.c

@@ -823,6 +823,7 @@ static struct net_device

*wwan_rtnl_alloc(struct nlattr *tb[],
        struct wwan_device *wwandev =
wwan_dev_get_by_name(devname);
        struct net_device *dev;
        unsigned int priv_size;
+       unsigned int num_txqs, num_rxqs;

        if (IS_ERR(wwandev))
                return ERR_CAST(wwandev);

@@ -833,9 +834,31 @@ static struct net_device

*wwan_rtnl_alloc(struct nlattr *tb[],
                goto out;
        }

+       /* let wwan device driver determine TX queue number if it
wants */
+       if (wwandev->ops->get_num_tx_queues) {
+               num_txqs = wwandev->ops-

quoted

get_num_tx_queues(num_tx_queues);

+               if (num_txqs < 1 || num_txqs > 4096) {
+                       dev = ERR_PTR(-EINVAL);
+                       goto out;
+               }
+       } else {
+               num_txqs = num_tx_queues;
+       }
+
+       /* let wwan device driver determine RX queue number if it
wants */
+       if (wwandev->ops->get_num_rx_queues) {
+               num_rxqs = wwandev->ops-

quoted

get_num_rx_queues(num_rx_queues);

+               if (num_rxqs < 1 || num_rxqs > 4096) {
+                       dev = ERR_PTR(-EINVAL);
+                       goto out;
+               }
+       } else {
+               num_rxqs = num_rx_queues;
+       }
+
        priv_size = sizeof(struct wwan_netdev_priv) + wwandev->ops-

quoted

priv_size;

        dev = alloc_netdev_mqs(priv_size, ifname, name_assign_type,
-                              wwandev->ops->setup, num_tx_queues,
num_rx_queues);
+                              wwandev->ops->setup, num_txqs,
num_rxqs);

        if (dev) {
                SET_NETDEV_DEV(dev, &wwandev->dev);

diff --git a/include/linux/wwan.h b/include/linux/wwan.h
index 9fac819f92e3..69c0af7ab6af 100644
--- a/include/linux/wwan.h
+++ b/include/linux/wwan.h

@@ -156,6 +156,10 @@ static inline void *wwan_netdev_drvpriv(struct

net_device *dev)
  * @setup: set up a new netdev
  * @newlink: register the new netdev
  * @dellink: remove the given netdev
+ * @get_num_tx_queues: determine number of transmit queues
+ *                     to create when creating a new device.
+ * @get_num_rx_queues: determine number of receive queues
+ *                     to create when creating a new device.
  */
 struct wwan_ops {
        unsigned int priv_size;

@@ -164,6 +168,8 @@ struct wwan_ops {
                       u32 if_id, struct netlink_ext_ack *extack);
        void (*dellink)(void *ctxt, struct net_device *dev,
                        struct list_head *head);
+       unsigned int (*get_num_tx_queues)(unsigned int hint_num);
+       unsigned int (*get_num_rx_queues)(unsigned int hint_num);
 };

 int wwan_register_ops(struct device *parent, const struct wwan_ops

*ops,

-- 
Sergey