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

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Message-ID: <20160617140512.GB1981@nanopsycho.orion>
Date:	Fri, 17 Jun 2016 16:05:13 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	David Ahern <dsa@...ulusnetworks.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	nogahf@...lanox.com, idosch@...lanox.com, eladr@...lanox.com,
	yotamg@...lanox.com, ogerlitz@...lanox.com,
	roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
	linville@...driver.com, tgraf@...g.ch, gospo@...ulusnetworks.com,
	sfeldma@...il.com, sd@...asysnail.net, eranbe@...lanox.com,
	ast@...mgrid.com, edumazet@...gle.com, hannes@...essinduktion.org,
	f.fainelli@...il.com
Subject: 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@...ulusnetworks.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.