netdev - Re: RPS will assign different smp_processor

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Date:	Sun, 24 Apr 2011 12:53:36 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	zhou rui <zhourui.cn@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RPS will assign different smp_processor_id for the same packet?

Le dimanche 24 avril 2011 à 17:36 +0800, zhou rui a écrit :
> >
> 
> my understanding:
> 
> non-NAPI scenario:
> 
> netif_rx( in irq, get_rps_cpu,enqueue_to_backlog to deliver packet to
> cpu queue) ------>net_rx_action(in softirq,deque and process packet)
> 
> 
> NAPI:
> 
> what does RPS do?(in
> irq)------------------------>net_rx_action(softirq)----->netif_receive_skb(get_rps_cpu,enque
> packet)
> 
> so my question is:
> for NAPI, get_rps_cpu will be done in softirq?
> 


> if the above situation is true,will this happen?
> packet_for_cpu_1 --> cpu0(netif_receive_skb,in softirq) --->delivered
> to cpu1(softirq)

NAPI is done under softirq, yes.

netif_receive_skb()
	cpu = get_rps_cpu()
	if (othercpu(cpu))
		enqueue_to_backlog(cpu)
	else
		__netif_receive_skb(skb);


enqueue_to_backlog() triggers an IPI (hard IRQ) to other cpu1,
to queue an NAPI context. (and triggers softirq)



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