Re: [RFC] x86_64: A real proposal for iret-less return to kernel

[Andy Lutomirski]

On Wed, May 21, 2014 at 3:25 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>
> Seems like a lot of effort and risk to essentially only optimize in kernel
> interrupt handlers.

The idea is that it might allow us to remove a bunch of scary nested
NMI code as well as speeding things up.

>
> AFAIK the most interesting cases (like user page faults) are not
> affected at all. Usually most workloads don't spend all that much time
> in the kernel, so it won't help most interrupts.
>
> I suspect the only case that's really interesting here is interrupting
> idle. Maybe it would be possible to do some fast path in this case only.
>
> However idle currently has so much overhead that I suspect that there
> are lower hanging fruit elsewhere.

I will gladly buy a meal or beverage for whomever fixes the ttwu stuff
to stop sending IPIs to idle CPUs, which will help a lot.

--Andy
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