Re: [PATCH v2 net-next 21/26] ice: add XDP and XSK generic per-channel statistics

[Jakub Kicinski]

On Mon, 29 Nov 2021 16:51:02 +0100 Petr Machata wrote:
> Jakub Kicinski <kuba@xxxxxxxxxx> writes:
> > On Sun, 28 Nov 2021 19:54:53 +0200 Ido Schimmel wrote:  
> >> I agree. I think IFLA_STATS64 in RTM_NEWLINK should contain statistics
> >> of all the packets seen by the netdev. The breakdown into software /
> >> hardware / XDP should be reported via RTM_NEWSTATS.  
> >
> > Hm, in the offload case "seen by the netdev" may be unclear. For 
> > the offload case I believe our recommendation was phrased more like 
> > "all packets which would be seen by the netdev if there was no
> > routing/tc offload", right?  
> 
> Yes. The idea is to expose to Linux stats about traffic at conceptually
> corresponding objects in the HW.

Great.

> >> Currently, for soft devices such as VLANs, bridges and GRE, user space
> >> only sees statistics of packets forwarded by software, which is quite
> >> useless when forwarding is offloaded from the kernel to hardware.
> >> 
> >> Petr is working on exposing hardware statistics for such devices via
> >> rtnetlink. Unlike XDP (?), we need to be able to let user space enable /
> >> disable hardware statistics as we have a limited number of hardware
> >> counters and they can also reduce the bandwidth when enabled. We are
> >> thinking of adding a new RTM_SETSTATS for that:
> >> 
> >> # ip stats set dev swp1 hw_stats on  
> >
> > Does it belong on the switch port? Not the netdev we want to track?  
> 
> Yes, it does, and is designed that way. That was just muscle memory
> typing that "swp1" above :)
> 
> You would do e.g. "ip stats set dev swp1.200 hw_stats on" or, "dev br1",
> or something like that.

I see :)

> >> For query, something like (under discussion):
> >> 
> >> # ip stats show dev swp1 // all groups
> >> # ip stats show dev swp1 group link
> >> # ip stats show dev swp1 group offload // all sub-groups
> >> # ip stats show dev swp1 group offload sub-group cpu
> >> # ip stats show dev swp1 group offload sub-group hw
> >> 
> >> Like other iproute2 commands, these follow the nesting of the
> >> RTM_{NEW,GET}STATS uAPI.  
> >
> > But we do have IFLA_STATS_LINK_OFFLOAD_XSTATS, isn't it effectively 
> > the same use case?  
> 
> IFLA_STATS_LINK_OFFLOAD_XSTATS is a nest. Currently it carries just
> CPU_HIT stats. The idea is to carry HW stats as well in that group.

Hm, the expectation was that the HW stats == total - SW. I believe that
still holds true for you, even if HW stats are not "complete" (e.g.
user enabled them after device was already forwarding for a while).
Is the concern about backward compat or such?