netdev - Re: [PATCH net-next] net: avoid unconditionally touching sk

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Message-ID: <67abfad3bec90_155892294aa@willemb.c.googlers.com.notmuch>
Date: Tue, 11 Feb 2025 20:35:15 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Paolo Abeni <pabeni@...hat.com>, 
 netdev@...r.kernel.org
Cc: Eric Dumazet <edumazet@...gle.com>, 
 Kuniyuki Iwashima <kuniyu@...zon.com>, 
 Willem de Bruijn <willemb@...gle.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next] net: avoid unconditionally touching sk_tsflags
 on RX

Paolo Abeni wrote:
> 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@...hat.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>