[PATCH net-next] net: Functions to report space available in device TX queues

[PATCH net-next] net: Functions to report space available in device TX queues

[Tom Herbert]

This patch adds netdev_tx_avail_queue and netdev_avail_queue which are
used to report number of bytes available in transmit queues per BQL. The
functions call dql_avail which returns BQL limit minus number of
inflight bytes. These functions can be called without txlock, for
instance to ascertain how much data should be dequeued from a qdisc in
a batch. When called without the tx_lock, the result is technically a
hint, subsequently when the tx_lock is done for a transmit it is
possible the availability has changed (for example a transmit
completion may have freed up more space in the queue or changed the
limit).

Signed-off-by: Tom Herbert <therbert@google.com>
---
 include/linux/netdevice.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0fac884..13883c1 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2544,6 +2544,30 @@ static inline void netdev_completed_queue(struct net_device *dev,
 	netdev_tx_completed_queue(netdev_get_tx_queue(dev, 0), pkts, bytes);
 }
 
+static inline int netdev_tx_avail_queue(struct netdev_queue *dev_queue)
+{
+#ifdef CONFIG_BQL
+	return dql_avail(&dev_queue->dql);
+#else
+	return DQL_MAX_LIMIT;
+#endif
+}
+
+/**
+ *	netdev_avail_queue - report how much space is availble for xmit
+ *	@dev: network device
+ *
+ *	Report the amount of space available in the TX queue in terms of
+ *	number of bytes. This returns the number of bytes avaiable per
+ *	DQL. This function may be called without taking the txlock on
+ *	the device, however in that case the result should be taken as
+ *	a (strong) hint.
+ */
+static inline int netdev_avail_queue(struct net_device *dev_queue)
+{
+	return netdev_tx_avail_queue(netdev_get_tx_queue(dev_queue, 0));
+}
+
 static inline void netdev_tx_reset_queue(struct netdev_queue *q)
 {
 #ifdef CONFIG_BQL
-- 
2.1.0.rc2.206.gedb03e5

Re: [PATCH net-next] net: Functions to report space available in device TX queues

[David Miller]

From: Tom Herbert <therbert@google.com>
Date: Sun, 24 Aug 2014 13:19:47 -0700 (PDT)

> +/**
> + *	netdev_avail_queue - report how much space is availble for xmit
> + *	@dev: network device
> + *
> + *	Report the amount of space available in the TX queue in terms of
> + *	number of bytes. This returns the number of bytes avaiable per
> + *	DQL. This function may be called without taking the txlock on
> + *	the device, however in that case the result should be taken as
> + *	a (strong) hint.
> + */
> +static inline int netdev_avail_queue(struct net_device *dev_queue)
> +{
> +	return netdev_tx_avail_queue(netdev_get_tx_queue(dev_queue, 0));
> +}
> +

This doesn't make any sense, you're only providing queue zero's
information.

You're passing in a net_device, calling it a "dev_queue" in the
variable name, the exlicitly using queue zero of that device in the
netdev_get_tx_queue() call.

Pretty confusing if you ask me :)

Re: [PATCH net-next] net: Functions to report space available in device TX queues

[Tom Herbert]

On Sun, Aug 24, 2014 at 10:35 PM, David Miller <davem@davemloft.net> wrote:
> From: Tom Herbert <therbert@google.com>
> Date: Sun, 24 Aug 2014 13:19:47 -0700 (PDT)
>
>> +/**
>> + *   netdev_avail_queue - report how much space is availble for xmit
>> + *   @dev: network device
>> + *
>> + *   Report the amount of space available in the TX queue in terms of
>> + *   number of bytes. This returns the number of bytes avaiable per
>> + *   DQL. This function may be called without taking the txlock on
>> + *   the device, however in that case the result should be taken as
>> + *   a (strong) hint.
>> + */
>> +static inline int netdev_avail_queue(struct net_device *dev_queue)
>> +{
>> +     return netdev_tx_avail_queue(netdev_get_tx_queue(dev_queue, 0));
>> +}
>> +
>
> This doesn't make any sense, you're only providing queue zero's
> information.
>
It's consistent with the other BQL functions. netdev_*_queue is used
for single queue devices, netdev_tx_*_queue is used for specific
queues in MQ.

> You're passing in a net_device, calling it a "dev_queue" in the
> variable name, the exlicitly using queue zero of that device in the
> netdev_get_tx_queue() call.
>
Cut and paste prototype from netdev_reset_queue which was doing
that... I'll fix it.

> Pretty confusing if you ask me :)

Re: [PATCH net-next] net: Functions to report space available in device TX queues

[Cong Wang]

On Mon, Aug 25, 2014 at 8:18 AM, Tom Herbert <therbert@google.com> wrote:
> On Sun, Aug 24, 2014 at 10:35 PM, David Miller <davem@davemloft.net> wrote:
>> From: Tom Herbert <therbert@google.com>
>> Date: Sun, 24 Aug 2014 13:19:47 -0700 (PDT)
>>
>>> +/**
>>> + *   netdev_avail_queue - report how much space is availble for xmit
>>> + *   @dev: network device
>>> + *
>>> + *   Report the amount of space available in the TX queue in terms of
>>> + *   number of bytes. This returns the number of bytes avaiable per
>>> + *   DQL. This function may be called without taking the txlock on
>>> + *   the device, however in that case the result should be taken as
>>> + *   a (strong) hint.
>>> + */
>>> +static inline int netdev_avail_queue(struct net_device *dev_queue)
>>> +{
>>> +     return netdev_tx_avail_queue(netdev_get_tx_queue(dev_queue, 0));
>>> +}
>>> +
>>
>> This doesn't make any sense, you're only providing queue zero's
>> information.
>>
> It's consistent with the other BQL functions. netdev_*_queue is used
> for single queue devices, netdev_tx_*_queue is used for specific
> queues in MQ.
>

I was also confused by these names when I read the code, it doesn't
make sense to me either. We need some cleanup no matter of your
patch. I will try to do it after your patch is in.

Re: [PATCH net-next] net: Functions to report space available in device TX queues

[Tom Herbert]

On Mon, Aug 25, 2014 at 12:24 PM, Cong Wang <cwang@twopensource.com> wrote:
> On Mon, Aug 25, 2014 at 8:18 AM, Tom Herbert <therbert@google.com> wrote:
>> On Sun, Aug 24, 2014 at 10:35 PM, David Miller <davem@davemloft.net> wrote:
>>> From: Tom Herbert <therbert@google.com>
>>> Date: Sun, 24 Aug 2014 13:19:47 -0700 (PDT)
>>>
>>>> +/**
>>>> + *   netdev_avail_queue - report how much space is availble for xmit
>>>> + *   @dev: network device
>>>> + *
>>>> + *   Report the amount of space available in the TX queue in terms of
>>>> + *   number of bytes. This returns the number of bytes avaiable per
>>>> + *   DQL. This function may be called without taking the txlock on
>>>> + *   the device, however in that case the result should be taken as
>>>> + *   a (strong) hint.
>>>> + */
>>>> +static inline int netdev_avail_queue(struct net_device *dev_queue)
>>>> +{
>>>> +     return netdev_tx_avail_queue(netdev_get_tx_queue(dev_queue, 0));
>>>> +}
>>>> +
>>>
>>> This doesn't make any sense, you're only providing queue zero's
>>> information.
>>>
>> It's consistent with the other BQL functions. netdev_*_queue is used
>> for single queue devices, netdev_tx_*_queue is used for specific
>> queues in MQ.
>>
>
> I was also confused by these names when I read the code, it doesn't
> make sense to me either. We need some cleanup no matter of your
> patch. I will try to do it after your patch is in.

That would be great if you can clean this up in a sane way, but beware
that some of these functions following the naming convention are
already in widespread use. For instance, both netif_stop_queue and
netif_tx_stop_queue are called a lot from drivers.