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

Fri, Jun 17, 2016 at 03:48:35PM CEST, dsa@cumulusnetworks.com wrote:

On 6/17/16 2:24 AM, Jiri Pirko wrote:

quoted

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.