netdev - Re: [PATCH] net: release skb->dst in sock_queue_rcv

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Date:	Tue, 25 Nov 2008 18:04:04 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	andi@...stfloor.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net: release skb->dst in sock_queue_rcv_skb()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Wed, 26 Nov 2008 01:00:30 +0100

> In the meantime, what do you think of the following patch ?
> 
> [PATCH] net: release skb->dst in sock_queue_rcv_skb()
> 
> When queuing a skb to sk->sk_receive_queue, we can release its dst, not
> anymore needed.
> Since current cpu did the dst_hold(), refcount is probably still hot
> int this cpu caches.
> 
> This avoids readers to access the original dst to decrement its refcount,
> possibly a long time after packet reception. This should speedup UDP
> and RAW receive path.
> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

I guess the idea is that if we release quickly we'll not have
to reget the cacheline in owned state.

I wonder if this might actually slightly hurt loads like tbench where
we are banging on the refcnt constantly on every cpu anyways.

Can you do a quick check?
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