Re: [patch net-next v5 0/4] return offloaded stats as default and expose original sw stats

On Tue, Jun 21, 2016 at 8:15 AM, Jiri Pirko [off-list ref] wrote:

From: Jiri Pirko <redacted>

The problem we try to handle is 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 the slow-path (through kernel).

Also note that HW stats are what someone calls "accumulated" stats.
Every packet counted by SW is also counted by HW. Not the other way around.

As a default the accumulated stats (HW) will be exposed to user
so the userspace apps can react properly.

curious, how do you plan to handle virtual device counters like vlan
and vxlan stats ?.
we can't separate CPU and HW stats there. In some cases (or ASICs) HW
counters do
not include CPU generated packets....you will have to add CPU
generated pkt counters to the
hw counters for such virtual device stats.

example: In the switchdev model, for bridge vlan stats, when user
queries bridge vlan stats,
you will have to add the hw stats to the bridge driver vlan stats and
return it to the user .

Having a consistent model for all kinds of stats will help.