netdev - Re: [PATCH net-next 3/3] udp: try to avoid 2 cache miss on dequeue

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Message-ID: <1496250276.27480.9.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 31 May 2017 10:04:36 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 3/3] udp: try to avoid 2 cache miss on dequeue

On Mon, 2017-05-29 at 17:27 +0200, Paolo Abeni wrote:
> when udp_recvmsg() is executed, on x86_64 and other archs, most skb
> fields are on cold cachelines.
> If the skb are linear and the kernel don't need to compute the udp
> csum, only a handful of skb fields are required by udp_recvmsg().
> Since we already use skb->dev_scratch to cache hot data, and
> there are 32 bits unused on 64 bit archs, use such field to cache
> as much data as we can, and try to prefetch on dequeue the relevant
> fields that are left out.
> 
> This can save up to 2 cache miss per packet.

okay ;)

> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/ipv4/udp.c | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 103 insertions(+), 11 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 53fa48d..616132e 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1163,6 +1163,83 @@ int udp_sendpage(struct sock *sk, struct page *page, int offset,
>  	return ret;
>  }
>  
> +/* Copy as much information as possible into skb->dev_scratch to avoid
> + * possibly multiple cache miss on dequeue();
> + */
> +#if BITS_PER_LONG == 64
> +
> +/* we can store multiple info here: truesize, len and the bit needed to
> + * compute skb_csum_unnecessary will be on cold cache lines at recvmsg
> + * time.
> + * skb->len can be stored on 16 bits since the udp header has been already
> + * validated and pulled.
> + */
> +struct udp_dev_scratch {
> +	__u32 truesize;
> +	__u16 len;
> +	__u16 is_linear:1;
> +	__u16 csum_unnecessary:1;

What about 
	u32   truesize;
	u16   len;
	bool  is_linear;
	bool  csum_unnecessary;

I do not believe the __ prefix is necessary for a local structure (not
uapi)

Also a plain bool or u8 is faster than a bit field (shorter
instructions)

Thanks.