net: avoid unconditionally touching sk_tsflags on RX

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 <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sock.h

···
 	SOCK_TSTAMP_NEW, /* Indicates 64 bit timestamps always */
 	SOCK_RCVMARK, /* Receive SO_MARK  ancillary data with packet */
 	SOCK_RCVPRIORITY, /* Receive SO_PRIORITY ancillary data with packet */
+	SOCK_TIMESTAMPING_ANY, /* Copy of sk_tsflags & TSFLAGS_ANY */
 };
 
 #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))
···
 {
 #define FLAGS_RECV_CMSGS ((1UL << SOCK_RXQ_OVFL)			| \
 			   (1UL << SOCK_RCVTSTAMP)			| \
-			   (1UL << SOCK_RCVMARK)			|\
-			   (1UL << SOCK_RCVPRIORITY))
+			   (1UL << SOCK_RCVMARK)			| \
+			   (1UL << SOCK_RCVPRIORITY)			| \
+			   (1UL << SOCK_TIMESTAMPING_ANY))
 #define TSFLAGS_ANY	  (SOF_TIMESTAMPING_SOFTWARE			| \
 			   SOF_TIMESTAMPING_RAW_HARDWARE)
 
-	if (sk->sk_flags & FLAGS_RECV_CMSGS ||
-	    READ_ONCE(sk->sk_tsflags) & TSFLAGS_ANY)
+	if (READ_ONCE(sk->sk_flags) & FLAGS_RECV_CMSGS)
 		__sock_recv_cmsgs(msg, sk, skb);
 	else if (unlikely(sock_flag(sk, SOCK_TIMESTAMP)))
 		sock_write_timestamp(sk, skb->tstamp);

net/core/sock.c

···
 
 	WRITE_ONCE(sk->sk_tsflags, val);
 	sock_valbool_flag(sk, SOCK_TSTAMP_NEW, optname == SO_TIMESTAMPING_NEW);
+	sock_valbool_flag(sk, SOCK_TIMESTAMPING_ANY, !!(val & TSFLAGS_ANY));
 
 	if (val & SOF_TIMESTAMPING_RX_SOFTWARE)
 		sock_enable_timestamp(sk,