netdev - Re: [RFC PATCH 1/5] net: implement support for low latency socket polling

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Date:	Sun, 03 Mar 2013 10:35:04 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.jf.intel.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Dave Miller <davem@...emloft.net>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	e1000-devel@...ts.sourceforge.net,
	Willem de Bruijn <willemb@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [RFC PATCH 1/5] net: implement support for low latency socket
 polling

On Wed, 2013-02-27 at 09:55 -0800, Eliezer Tamir wrote:

> index 821c7f4..d1d1016 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -408,6 +408,10 @@ struct sk_buff {
>  	struct sock		*sk;
>  	struct net_device	*dev;
>  
> +#ifdef CONFIG_INET_LL_RX_POLL
> +	struct napi_struct	*dev_ref; /* where this skb came from */
> +#endif
> +
>  	/*
>  	 * This is the control buffer. It is free to use for every
>  	 * layer. Please put your private variables there. If you

Yes, thats the killer, because :

1) It adds 8 bytes per skb, and we are going to reach the 256 bytes per
sk_buff boundary. cloned skbs will use an extra cache line.

It might make sense to union this on dma_cookie, as dma_cookie is only
used on TX path.

2) We need to reference count napi structs.

For 2) , we would need to add a percpu ref counter (a bit like struct
netdevice -> pcpu_refcnt)

Alternative to 2) would be to use a generation id, incremented every
time a napi used in spin polling enabled driver is dismantled (and freed
after RCU grace period)

And store in sockets not only the pointer to napi_struct, but the
current generation id : If the generation id doesnt match, disable
the spinpoll until next packet rebuilds the cache again.



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