Re: [patch net-next v4 0/4] return offloaded stats as default and expose original sw stats

Jiri Pirko
2016-06-17T14:05:16Z

Fri, Jun 17, 2016 at 03:48:35PM CEST, dsa@cumulusnetworks.com wrote:
>On 6/17/16 2:24 AM, Jiri Pirko wrote:
>>
>>The problem we try to handle is different, it's about offloaded
>>forwarded packets which are not seen by kernel. Let me try to draw it :)
>>
>> port1 port2 (HW stats are counted here)
>> \ /
>> \ /
>> \ /
>> --(A)---- ASIC --(B)--
>> |
>> (C)
>> |
>> CPU (SW stats are counted here)
>>
>>
>>Now we have couple of flows for TX and RX (direction does not matter here):
>>
>>1) port1->A->ASIC->C->CPU
>>
>> For this flow, HW and SW stats are equal.
>>
>>2) port1->A->ASIC->C->CPU->C->ASIC->B->port2
>>
>> For this flow, HW and SW stats are equal.
>>
>>3) port1->A->ASIC->B->port2
>>
>> For this flow, SW stats are 0.
>>
>>The purpose of this patchset is to provide facility for user to
>>find out the difference between flows 1+2 and 3. In other words, user
>>will be able to see the statistics for his slow-path (through kernel).
>>
>>Also, as a default the accumulated stats (HW) will be exposed to user
>>so the userspace apps can react properly.
>>
>
>You no longer agree with this discussion?
> http://comments.gmane.org/gmane.linux.network/346740
>
>Essentially netdevice stats show counters for packets punted to the cpu and
>ethool -S shows h/w stats. This patch set seems to invert that.

That is problematic. Existing apps depend on rtnetlink stats. But if we
don't count offloaded forwarded packets, the apps don't see anything.
Therefore I believe that this patchset approach is better. The existing
apps continue to work and future apps can use newly introduces sw_stats
to query slowpath traffic. Makes sense to me.