netdev - Re: [PATCH net-next] net: sock_rps_record

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Date:   Wed, 07 Dec 2016 07:59:32 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net-next] net: sock_rps_record_flow() is for connected
 sockets

On Wed, 2016-12-07 at 06:29 -0800, Eric Dumazet wrote:

> Keeping frag_list means you keep one sk_buff per segment, so this really
> looks like a legacy UDP server (like a DNS server) wont benefit from
> this anyway.

I played with the idea of preparing the skb for minimal overhead for the
process doing udp_recvmsg().

If socket is under pressure, softirq handler(s) can try to pull in
skb->head the payload of the packet if it fits.

Meaning the softirq handler can free/reuse the page fragment
immediately, instead of letting udp_recvmsg() do this hundreds of usec
later.

( Sort of copybreak, but without reallocating skb->head )

Gains :
- We reduce skb->truesize and thus can store more packets per SO_RCVBUF
 KB
- We avoid a cache line misses at copyout() time and consume_skb() time,
and avoid one put_page() with potential alien freeing on NUMA hosts.