netdev - Re: Setting RPS affinities from network driver

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Date:	Fri, 20 Feb 2015 17:46:25 -0500
From:	Jonathon Reinhart <jonathon.reinhart@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Sunil Kovvuri <sunil.kovvuri@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Setting RPS affinities from network driver

On Fri, Feb 20, 2015 at 5:30 PM, Tom Herbert <therbert@...gle.com> wrote:
> On Thu, Feb 19, 2015 at 6:07 PM, Jonathon Reinhart
> <jonathon.reinhart@...il.com> wrote:
>>
>> I don't know if this is a good idea. It seems like allowing the driver to
>> set this default configuration opens the door to a whole slew of driver-
>> specific config customizations.
>>
>> As as user, I wouldn't expect one driver to have a different default
>> value of rps_cpus than another driver. Furthermore, I could imagine a
>> case where a user expects rps_cpus to default to zero for all of his
>> NICs, and has other CPU affinity settings applied (think realtime).
>>
>> In my opinion, this should be left to userspace. It's not that hard to
>> add it to your init scripts.
>>
>>
> In the old days I might have agreed with that-- kernel implements
> mechanism and policy is handled in user space. However, in this brave
> new world of hardware offloads for more and more kernel networking,
> I'm not so sure about this any more. It's starting to look like we may
> want kernel to do more dynamic resource management under some
> described policies.
>
> In davem's keynote at netdev he mentioned that hardware offload should
> be transparent to things like ip route, so one implication is that at
> some point the kernel may need to decide which routes are the best to
> offload per a policy. Also at netdev, Jesse Brandeburg mentioned that
> Windows had a capabilities of spinning up RSS queues to handle
> increased load, and lamented that we couldn't do this easily in Linux.
> Willem deBruijn made RPS resilient under DOS with RPS flow limit as a
> step in that direction. We have spent countless hours tuning RPS and
> interrupt settings per platform, per NIC, and per some major
> applications-- but the resultant init scripts that result are very
> convoluted and static. It is an interesting idea if we could just tell
> kernel to take it's best guess and be adaptive. I suppose something
> like irqbalance is the alternative to do this sort of stuff in user
> space, but I don't know if that is well deployed or sufficiently
> reactive under DOS attack.

I agree with your general sentiment, but does it make sense for this to
be controlled at a per-driver level? The OP indicates that he wants his
driver to perform well, so he wants to enable RPS for it by default. If the
RPS policy is to go into the kernel, shouldn't it be done at the global
scope?

Note that this argument is different from RSS where we're dealing with
actual hardware queues, so the driver of course has a say in the
configuration.

Going on with what you alluded to, I think if the kernel were to have a say
in the RPS configuration, there would need to be some sort of global on/off
switch.
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