netdev - Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

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Date:   Tue, 06 Dec 2016 10:42:37 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     pabeni@...hat.com, netdev@...r.kernel.org, willemb@...gle.com
Subject: Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 05 Dec 2016 09:57:19 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> In UDP recvmsg() path we currently access 3 cache lines from an skb
> while holding receive queue lock, plus another one if packet is
> dequeued, since we need to change skb->next->prev
> 
> 1st cache line (contains ->next/prev pointers, offsets 0x00 and 0x08)
> 2nd cache line (skb->len & skb->peeked, offsets 0x80 and 0x8e)
> 3rd cache line (skb->truesize/users, offsets 0xe0 and 0xe4)
> 
> skb->peeked is only needed to make sure 0-length packets are properly
> handled while MSG_PEEK is operated.
> 
> I had first the intent to remove skb->peeked but the "MSG_PEEK at
> non-zero offset" support added by Sam Kumar makes this not possible.
> 
> This patch avoids one cache line miss during the locked section, when
> skb->len and skb->peeked do not have to be read.
> 
> It also avoids the skb_set_peeked() cost for non empty UDP datagrams.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.