netdev - RE: [PATCH net-next] udp: use sk_protocol instead of pcflag to detect udplite sockets

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Date:   Fri, 31 Mar 2017 15:03:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Paolo Abeni <pabeni@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net-next] udp: use sk_protocol instead of pcflag to
 detect udplite sockets

From: Eric Dumazet
> Sent: 31 March 2017 14:25
> On Fri, 2017-03-31 at 11:47 +0200, Paolo Abeni wrote:
> > In the udp_sock struct, the 'forward_deficit' and 'pcflag' fields
> > share the same cacheline. While the first is dirtied by
> > udp_recvmsg, the latter is read, possibly several times, by the
> > bottom half processing to discriminate between udp and udplite
> > sockets.
> >
> > With this patch, sk->sk_protocol is used to check is the socket is
> > really an udplite one, avoiding some cache misses per
> > packet and improving the performance under udp_flood with
> > small packet up to 10%.
...
> I am pretty sure we agreed in the past that forward_deficit would need
> to be placed on a cache line of its own. Somehow we manage to not
> implement this properly.
> 
> What about other fields like encap_rcv, encap_destroy, gro_receive,
> gro_complete. They really should have the same false sharing issue.
> 
> Proper fix is :
...
> -	/* This field is dirtied by udp_recvmsg() */
> -	int		forward_deficit;
> +	/* This field is dirtied by udp_recvmsg().
> +	 * Make sure it wont share a cache line with prior fields.
> +	 */
> +	int		forward_deficit ____cacheline_aligned_in_smp;

Is that really sensible on systems with large cache lines?

	David