netdev - Re: RPS will assign different smp_processor

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Date:	Sat, 23 Apr 2011 23:31:37 +0800
From:	zhou rui <zhourui.cn@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RPS will assign different smp_processor_id for the same packet?

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?

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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