netdev - Re: RPS will assign different smp_processor

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Message-ID: <1303403112.3685.61.camel@edumazet-laptop>
Date:	Thu, 21 Apr 2011 18:25:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	zhou rui <zhourui.cn@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: RPS will assign different smp_processor_id for the same packet?

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"

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