Re: [PATCH] net/af_packet: cache align Rx/Tx structs

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 25 Apr 2024 00:27:36 +0200
Mattias Rönnblom <hofors@lysator.liu.se> wrote:

> On 2024-04-24 21:13, Stephen Hemminger wrote:
> > On Wed, 24 Apr 2024 18:50:50 +0100
> > Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> >   
> >>> I don't know how slow af_packet is, but if you care about performance,
> >>> you don't want to use atomic add for statistics.
> >>>      
> >>
> >> There are a few soft drivers already using atomics adds for updating stats.
> >> If we document expectations from 'rte_eth_stats_reset()', we can update
> >> those usages.  
> > 
> > Using atomic add is lots of extra overhead. The statistics are not guaranteed
> > to be perfect.  If nothing else, the bytes and packets can be skewed.
> >   
> 
> The sad thing here is that in case the counters are reset within the 
> load-modify-store cycle of the lcore counter update, the reset may end 
> up being a nop. So, it's not like you missed a packet or two, or suffer 
> some transient inconsistency, but you completed and permanently ignored 
> the reset request.

That is one of the many reasons the Linux kernel intentionally did not
implement a reset statistics operation.