[ltt-dev] net isntrumentation: duplicated receive

[Mathieu Desnoyers]

* 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).

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