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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> Sent: Thursday, 25 April 2024 11.26
> 
> On 2024-04-25 01:55, Stephen Hemminger wrote:
> > 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.
> 
> DPDK should deprecate statistics reset, it seems to me.

+1

> 
> The current API is broken anyway, if you care about correctness. A reset
> function must return the current state of the counters, at the point
> them being reset. Otherwise, a higher layer may miss counter updates.
> 
> The af_packet PMD should return -ENOTSUP on reset (which is allowed by
> the API).
> 
> Maintaining an offset-since-last-reset for counters is a control plane
> thing, and shouldn't be in PMDs. (If MT-safe reset for SW-managed
> counters are to be expected from the PMDs, we should have some helper
> API to facilitate its efficient & correct-enough implementation.)