netdev - Re: Fighting out-of-order reception with RPS?

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Date:	Sat, 11 Jul 2015 06:35:45 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
Cc:	Tom Herbert <tom@...bertland.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
	Sunil Kovvuri <sunil.kovvuri@...il.com>,
	Jonathon Reinhart <jonathon.reinhart@...il.com>
Subject: Re: Fighting out-of-order reception with RPS?

On Fri, 2015-07-10 at 22:36 +0200, Oliver Hartkopp wrote:
> On 07/10/2015 04:48 AM, Tom Herbert wrote:
> > On Wed, Jul 8, 2015 at 10:55 PM, Oliver Hartkopp <socketcan@...tkopp.net> wrote:
> >> Both drivers do not use NAPI. The just follow the way
> >>
> >> interrupt -> alloc_skb() -> fill skb -> netif_rx(skb)
> >>
> >> I'm usually testing with the USB adapters as the PCIe setup is not very
> >> handy.
> >>
> > Okay, I see what is happening. In netif_rx when RPS is not enabled
> > that packet is queued to the backlog queue for the local CPU. Since
> > you're doing round robin on the interrupts then OOO packets can be a
> > result. Unfortunately, this is the expected behavior. The correct
> > kernel fix would be to move to these drivers to use NAPI.
> 
> Hm. Doesn't sound like a good solution when there's a difference between NAPI
> and non-NAPI drivers in matters of OOO, right?

Isn't OOO a problem for you ? Then you either have to :

1) Use a single CPU to handle IRQ from the device
2) Use NAPI

> 
> > RPS
> > eliminates the OOO, but if there is no ability to derive a flow hash
> > from packets everything will wind up one queue without load balancing.
> 
> Correct.
> 
> That's why I added
> 
> 	skb_set_hash(skb, dev->ifindex, PKT_HASH_TYPE_L2);
> 
> in my driver, because the only relevant flow identifiction is the number of
> the incoming CAN interface.
> 
> > Besides that, automatically setting RPS in drivers is a difficult
> > proposition since there is no definitively "correct" way to do that in
> > an arbitrary configuration.
> 
> What about checking in netif_rx() if the non-NAPI driver has set a hash (aka
> the driver is OOO sensitive)?
> And if so we could automatically set rps_cpus for this interface in a way that
> all CPUs are enabled to take skbs following the hash.

Wow, netif_rx() is packet processing fast path, certainly not the place
to add controlling path decisions.

Please convert your driver to NAPI. You might then even benefit from
GRO.


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