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

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Message-ID: <CALCETrVambcEkh8oQuUL95DupX4Lhxwh5W0tESNqia26z3_z3g@mail.gmail.com>
Date:	Wed, 21 May 2014 15:32:33 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Wed, May 21, 2014 at 3:25 PM, Andi Kleen <andi@...stfloor.org> 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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