[ltt-dev] net isntrumentation: duplicated receive

[Mathieu Desnoyers]

* Benjamin Poirier (benjamin.poirier at polymtl.ca) wrote:
> On 24/10/10 09:38 PM, Mathieu Desnoyers wrote:
>> * Benjamin Poirier (benjamin.poirier at polymtl.ca) wrote:
>>> On 22/10/10 04:26 PM, Mathieu Desnoyers wrote:
>>>> Hi Benjamin,
>>>>
>>>> Can you explain why you need two receive tracepoints in the recv path ?
>>>
>>> One tracepoint is in netif_rx() and is fired by drivers that use the
>>> pre-NAPI receive path. The other tracepoint is in netif_receive_skb()
>>> and is fired by drivers that use the NAPI receive path.
>>>
>>>>
>>>> "The receive tracepoint is moved earlier in the netif_receive_skb() function.
>>>> The tracepoint is also duplicated in the netif_rx() function. This enables
>>>> more precise measurements for pre-NAPI drivers."
>>>>
>>>> @@ -2518,9 +2520,13 @@ int netif_rx(struct sk_buff *skb)
>>>>           if (netpoll_rx(skb))
>>>>                   return NET_RX_DROP;
>>>>
>>>> +       trace_net_dev_receive(skb);
>>>> +
>>>>           if (netdev_tstamp_prequeue)
>>>>                   net_timestamp_check(skb);
>>>>
>>>> +       trace_net_dev_receive(skb);
>>>>
>>>> The side-effect is that two events will be generated for each packet, no ?
>>>
>>> No. Depending on the receive path, only one of the two tracepoints is
>>> hit. netif_rx() is specific to pre-NAPI drivers. It eventually leads to
>>> __netif_receive_skb() and therefore avoids the tracepoint in
>>> netif_receive_skb(). NAPI drivers just don't call netif_rx() and
>>> therefore avoid the tracepoint in there.
>>>
>>> It would be possible to have only one tracepoint in
>>> __netif_receive_skb() but that would make the event be generated later
>>> on the receive path (in all cases). One of the goals when tracing packet
>>> reception is to get a timestamp as close as possible to the packet
>>> arrival.
>>>
>>> Hope this clears things up,
>>
>> Not really. What I am saying here is that the instrumentation in "netif_rx()"
>> really calls "trace_net_dev_receive(skb);" *twice*. Once before the call to
>> "net_timestamp_check(skb)" and once after. I really think it is an
>> implementation error (or maybe a patch merge error somewhere).
>
> Yeah, that patch hunk looks wonky. The one in the before-last version  
> (.232) of the kernel.org lttng repository is fine though.

OK, I'll fix it and release 0.234.

Thanks,

Mathieu

>
>>
>> So all pre-NAPI receive paths which call into netif_rx will call the
>> net_dev_receive event twice logically here, no ?
>>
>> Thanks,
>>
>> Mathieu
>>

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com