netdev - Re: [PATCH net-next 1/4] net: SO_INCOMING

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Date:	Thu, 08 Oct 2015 14:17:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/4] net: SO_INCOMING_CPU setsockopt() support

On Thu, 2015-10-08 at 13:53 -0700, Tom Herbert wrote:

> If the incoming CPU is set for a connected UDP via
> sk_incoming_cpu_update, wouldn't this check subsequently _only_ allow
> packets for that socket to come from the same CPU?
> 

Hmm, I thought the SO_REUSEPORT path would be taken only for non
connected UDP sockets (like TCP listeners.).

But you might be right !

> Also, the check seems a little austere. Why not do something like:
> 
>                if (sk->sk_incoming_cpu != -1) {
>                        if (sk->sk_incoming_cpu != raw_smp_processor_id())
>                             score += 4;
>                }
> 
> My worry is that the packet steering configuration may change without
> the application's knowledge, so it's possible packets may come in on
> CPUs that the are unexpected to the application and then they would be
> dropped without matching a socket. I suppose that this could work with
> the original patch if a socket is bound to every CPU or there is at
> least one listener socket that is not bound to any CPU.

This is what I initially wrote, then I attempted a short cut, (abort
full list scan), then forgot to re-instate the first try, when I decided
to let this for future patch (Ying patch)

if (sk->sk_incoming_cpu == raw_smp_processor_id())
    score++;

(Note we do not even have to test for sk_incoming_cpu == -1 in this
variant)

I'll include this in v2.

Thanks.


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