netdev - Re: RPS will assign different smp_processor

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Date:	Sun, 24 Apr 2011 17:36:51 +0800
From:	zhou rui <zhourui.cn@...il.com>
To:	Eric Dumazet <eric.dumazet@...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?

On Sun, Apr 24, 2011 at 4:00 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le dimanche 24 avril 2011 à 10:00 +0800, zhou rui a écrit :
>> On Sun, Apr 24, 2011 at 3:56 AM, Tom Herbert <therbert@...gle.com> wrote:
>> > On Sat, Apr 23, 2011 at 8:31 AM, zhou rui <zhourui.cn@...il.com> wrote:
>> >> one more question is:
>> >>
>> >> in the function "int netif_receive_skb(struct sk_buff *skb)"
>> >>
>> >> cpu = get_rps_cpu(skb->dev, skb, &rflow);
>> >> if (cpu >= 0) {
>> >>  ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
>> >> ....
>> >>
>> >> probably the cpu is different from the current processor id?(smp_processor_id)
>> >> let's say: get_rps_cpu->cpu 0, smp_processor_id->cpu1
>> >> when this happen, does it mean that cpu1 is handling the softirq but
>> >> have to divert the packet to cpu0?(via a new softirq?)
>> >>
>> >> so for one packet it involve 2 softirqs?
>> >>
>> >> possible to get_rps_cpu in interrupt,then let the target cpu do only
>> >> one softirq to hanle the packet?
>> >>
>> > Yes, this is what a non-NAPI driver would do.
>> >
>> > Tom
>> >
>> non-NAPI will get_rps_cpu in irq, but why NAPI will get_rps_cpu in
>> softirq?(if I understand correctly netif_receive_skb executed in
>> softirq?)
>
> Thats hard to understand what your problem or question is.
>
> All heavy Receive networking stuff is handled in softirq.
>
> NAPI allows to reduce number of hardware IRQS in stress/load situations,
> fetching several frames at once.
>
>
>
>

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)
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