Re: [PATCH net-next] net: avoid unconditionally touching sk_tsflags on RX

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Paolo Abeni <pabeni@redhat.com>
Date: Tue, 11 Feb 2025 18:17:31 +0100
> After commit 5d4cc87414c5 ("net: reorganize "struct sock" fields"),
> the sk_tsflags field shares the same cacheline with sk_forward_alloc.
> 
> The UDP protocol does not acquire the sock lock in the RX path;
> forward allocations are protected via the receive queue spinlock;
> additionally udp_recvmsg() calls sock_recv_cmsgs() unconditionally
> touching sk_tsflags on each packet reception.
> 
> Due to the above, under high packet rate traffic, when the BH and the
> user-space process run on different CPUs, UDP packet reception
> experiences a cache miss while accessing sk_tsflags.
> 
> The receive path doesn't strictly need to access the problematic field;
> change sock_set_timestamping() to maintain the relevant information
> in a newly allocated sk_flags bit, so that sock_recv_cmsgs() can
> take decisions accessing the latter field only.
> 
> With this patch applied, on an AMD epic server with i40e NICs, I
> measured a 10% performance improvement for small packets UDP flood
> performance tests - possibly a larger delta could be observed with more
> recent H/W.
> 
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>