netdev - Re: [RFC] rps: shortcut net_rps

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Message-ID: <1271679244.3845.43.camel@edumazet-laptop>
Date:	Mon, 19 Apr 2010 14:14:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] rps: shortcut net_rps_action()

Le lundi 19 avril 2010 à 17:48 +0800, Changli Gao a écrit :
> On Mon, Apr 19, 2010 at 5:37 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > net_rps_action() is a bit expensive on NR_CPUS=64..4096 kernels, even if
> > RPS is not active.
> >
> > I add a flag to scan cpumask only if at least one IPI was scheduled.
> > Even cpumask_weight() might be expensive on some setups, where
> > nr_cpumask_bits could be very big (4096 for example)
> 
> How about using a array to save the cpu IDs. The number of CPUs, to
> which the IPI will be sent, should be small.
> 

Yes it should be small, yet the two arrays would be big enough to make
softnet_data first part use at least two cache lines instead of one,
even in the case we handle one cpu/IPI per net_rps_action()

As several packets can be enqueued for a given cpu, we would need to
keep bitmasks.
We would have to add one test in enqueue_to_backlog()

if (cpu_test_and_set(cpu, mask)) {
	__raise_softirq_irqoff(NET_RX_SOFTIRQ);
	array[nb++] = cpu;
}



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