netdev - Re: [PATCH] Software receive packet steering

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Date:	Mon, 20 Apr 2009 20:26:13 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH] Software receive packet steering

On Mon, Apr 20, 2009 at 3:32 AM, Andi Kleen <andi@...stfloor.org> wrote:
>
> Tom Herbert <therbert@...gle.com> writes:
>
> > +static int netif_cpu_for_rps(struct net_device *dev, struct sk_buff *skb)
> > +{
> > +     cpumask_t mask;
> > +     unsigned int hash;
> > +     int cpu, count = 0;
> > +
> > +     cpus_and(mask, dev->soft_rps_cpus, cpu_online_map);
> > +     if (cpus_empty(mask))
> > +             return smp_processor_id();
>
> There's a race here with CPU hotunplug I think. When a CPU is hotunplugged
> in parallel you can still push packets to it even though they are not
> drained. You probably need some kind of drain callback in a CPU hotunplug
> notifier that eats all packets left over.
>
We will look at that, the hotplug support may very well be lacking in the patch.

> > +got_hash:
> > +     hash %= cpus_weight_nr(mask);
>
> That looks rather heavyweight even on modern CPUs. I bet it's 40-50+ cycles
> alone forth the hweight and the division. Surely that can be done better?
>
Agreed, I will try to pull in the RX hash from Dave Miller's remote
softirq patch.

> Also I suspect some kind of runtime switch for this would be useful.
>
> Also the manual set up of the receive mask seems really clumpsy. Couldn't
> you set that up dynamically based on where processes executing recvmsg()
> are running?
>
We have done exactly that.  It works very well in many cases
(application + platform combinations), but I haven't found it to be
better than doing the hash in all cases.  I could provide the patch,
but it might be more of a follow patch to this base one.

Thanks,
Tom
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