netdev - Re: RPS will assign different smp_processor

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Date:	Sun, 24 Apr 2011 10:00:33 +0800
From:	zhou rui <zhourui.cn@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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 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?)
thanks!

>
>> thanks
>>
>> On Fri, Apr 22, 2011 at 12:29 AM, zhou rui <zhourui.cn@...il.com> wrote:
>>> On Friday, April 22, 2011, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>> Le jeudi 21 avril 2011 à 18:08 +0200, Eric Dumazet a écrit :
>>>>> Le jeudi 21 avril 2011 à 23:50 +0800, zhou rui a écrit :
>>>>> > kernel 2.6.36.4
>>>>> > CONFIG_RPS=y but not set the cpu mask
>>>>> >
>>>>> > /sys/class/net/eth1/queues/rx-0 # cat rps_cpus
>>>>> > 00
>>>>> >
>>>>> > register a hook func:
>>>>> >   prot_hook.func = packet_rcv;
>>>>> >   prot_hook.type = htons(ETH_P_ALL);
>>>>> >   dev_add_pack(&prot_hook);
>>>>> >
>>>>> >
>>>>> > replay the same traffic in very slow speed, printk the
>>>>> > smp_processor_id in packet_rcv():
>>>>> > first time:
>>>>> > cpu=4
>>>>> > cpu=3
>>>>> > cpu=6
>>>>> > cpu=7
>>>>> >
>>>>> > second time:
>>>>> > cpu=7
>>>>> > cpu=1
>>>>> > cpu=5
>>>>> > cpu=2
>>>>> >
>>>>> > is it normal?
>>>>>
>>>>> Yes it is.
>>>>>
>>>>> What would you expect ?
>>>>>
>>>>
>>>> If rps_cpus contains only '0' bits, it basically means RPS is not active
>>>> for this input queue.
>>>>
>>>> CPU is therefore not changed : The cpu handling NAPI on your network
>>>> device directly calls upper linux stack.
>>>>
>>>> Seeing your traces, it also means your device spreads its interrupts on
>>>> many different cpus, this might be not optimal.
>>>>
>>>> Check /proc/irq/{irq_number}/smp_affinity, it probably contains "ff"
>>>>
>>>>
>>>>
>>>>
>>> Thanks,just saw this email
>>>
>> --
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>
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