On 7/22/2021 12:10 AM, Paolo Abeni wrote:

Hello,

On Wed, 2021-07-21 at 11:15 -0700, Casey Schaufler wrote:

quoted

On 7/21/2021 9:44 AM, Paolo Abeni wrote:

quoted

This is a very early draft - in a different world would be
replaced by hallway discussion at in-person conference - aimed at
outlining some ideas and collect feedback on the overall outlook.
There are still bugs to be fixed, more test and benchmark need, etc.

There are 3 main goals:
- [try to] avoid the overhead for uncommon conditions at GRO time
  (patches 1-4)
- enable backpressure for the veth GRO path (patches 5-6)
- reduce the number of cacheline used by the sk_buff lifecycle
  from 4 to 3, at least in some common scenarios (patches 1,7-9).
  The idea here is avoid the initialization of some fields and
  control their validity with a bitmask, as presented by at least
  Florian and Jesper in the past.

If I understand correctly, you're creating an optimized case
which excludes ct, secmark, vlan and UDP tunnel. Is this correct,
and if so, why those particular fields? What impact will this have
in the non-optimal (with any of the excluded fields) case?

Thank you for the feedback.

You're most welcome. You did request comments.

There are 2 different relevant points:

- the GRO stage.
  packets carring any of CT, dst, sk or skb_ext will do 2 additional
conditionals per gro_receive WRT the current code. My understanding is
that having any of such field set at GRO receive time is quite
exceptional for real nic. All others packet will do 4 or 5 less
conditionals, and will traverse a little less code.

- sk_buff lifecycle
  * packets carrying vlan and UDP will not see any differences: sk_buff
lifecycle will stil use 4 cachelines, as currently does, and no
additional conditional is introduced.
  * packets carring nfct or secmark will see an additional conditional
every time such field is accessed. The number of cacheline used will
still be 4, as in the current code. My understanding is that when such
access happens, there is already a relevant amount of "additional" code
to be executed, the conditional overhead should not be measurable.

I'm responsible for some of that "additonal" code. If the secmark
is considered to be outside the performance critical data there are
changes I would like to make that will substantially improve the
performance of that "additional" code that would include a u64
secmark. If use of a secmark is considered indicative of a "slow"
path, the rationale for restricting it to u32, that it might impact
the "usual" case performance, seems specious. I can't say that I
understand all the nuances and implications involved. It does
appear that the changes you've suggested could negate the classic
argument that requires the u32 secmark.

Cheers,

Paolo