netdev - Re: [PATCH RFC] net: save RX queue number in sock for dev_pick

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Date:	Tue, 24 Aug 2010 15:06:13 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>, Tom Herbert <therbert@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] net: save RX queue number in sock for
 dev_pick_tx() use

Le mardi 24 août 2010 à 17:01 +0800, Changli Gao a écrit :
> For the packets sent out from a local server socket, we can use the queue
> from which the packets from the client socket are received.
> 
> It may help on a TCP or UDP server. Because I don't have a multiqueue NIC,
> I don't even test it.
> 

It may help ? or it may not...

> Signed-off-by: Changli Gao <xiaosuo@...il.com>
> ---
>  include/net/sock.h  |   18 ++++++++++++++++++
>  net/core/dev.c      |   25 ++++++++++++++++++-------
>  net/ipv4/tcp_ipv4.c |    5 ++++-
>  net/ipv4/udp.c      |    4 +++-
>  4 files changed, 43 insertions(+), 9 deletions(-)
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 100e43b..4e5f2f4 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1231,6 +1231,24 @@ static inline int sk_tx_queue_get(const struct sock *sk)
>  	return sk ? sk->sk_tx_queue_mapping : -1;
>  }
>  
> +static inline void sk_rx_queue_save(struct sock *sk, struct sk_buff *skb)
> +{
> +	struct dst_entry *dst;
> +	int rxqueue;
> +
> +	if (!skb_rx_queue_recorded(skb))
> +		return;
> +	rcu_read_lock();
> +	dst = rcu_dereference_check(sk->sk_dst_cache, 1);

rcu_dereference(sk->sk_dst_cache) is fine here.

> +	if (dst && !dst->dev->netdev_ops->ndo_select_queue &&
> +	    dst->dev == skb->dev) {
> +		rxqueue = skb_get_rx_queue(skb);
> +		if (rxqueue != sk_tx_queue_get(sk))
> +			sk_tx_queue_set(sk, rxqueue);
> +	}
> +	rcu_read_unlock();
> +}
> +

ipv6 not handled

Anyway, should we store a queue number, a rxhash, a cpu number, all
values ?

Tom Herbert last patch (XPS) is about selecting a txqueue given a cpu
number at sending time. In its use, no need to add logic at receive time
to remember rxqueue. But logic is wide (for a given NIC)

I do think we should really clarify the needs before writing hundred of
lines of code that we have to maintain for next decade...

For example, storing the cpu number of the cpu that received a UDP/TCP
frame would probably be nice, and using it as a key for xmit path queue
selection would probably be faster...

Probably we want a selectable way... A multi threaded UDP application,
receiving frames on a single socket, might need to select a single
txqueue (socket property), or multiple queues (based on packet content,
or sender cpu, or last receive cpu number, ...)

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