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

Fri, Jun 17, 2016 at 02:26:32AM CEST, davem@davemloft.net wrote:

From: Jiri Pirko <jiri@resnulli.us>
Date: Thu, 16 Jun 2016 10:37:13 +0200

quoted

Until now we had stats functions return SW statistics. However, it makes
a lot of sense to return HW stats as default. The existing apps count with
having the defaults stats complete, but that is not true now as the offloaded
forward traffic is not visible there.

If user wants to know real SW stats, this patchset provides way to get
it as well.

I think we haven't been very good about defining good rules nor
guidelines for how to handle HW vs SW stats, and I'm talking
strictly about what we publish via rtnl_link_stats64.

However, if I were working from scratch on a new driver what I would
be inclined to do is use HW stats for everything that the chip
provides direct and accurate support for, and fill in the gaps with SW
stats.

Because to me, stats are stats, the user wants to know (for example)
how many broadcast packets have gone through the port and doesn't care
how you obtain that number.

If the problem being addressed is that drivers aren't reporting
information on all the packets going through the device, then that's a
bug.

But it seems to me like mlxsw is already maintaining the software
statistic counters, so I can't see a performance reason for not
properly providing all of the statistics using HW vs. SW as is
appropriate for each and every value to fix this problem.  Why
create an entire new facility just for that?  It doesn't seem to
be needed.

Maybe you just need to describe things a bit more completely in this
header posting.

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.