linux-kernel - Re: [PATCH 3/4] x86: save user rsp in pt

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Message-ID: <20150310132606.GC26185@gmail.com>
Date:	Tue, 10 Mar 2015 14:26:06 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64
 fastpath


* Andy Lutomirski <luto@...capital.net> wrote:

> > Since this patch does add two extra MOVs,
> > I did benchmark these patches. They add exactly one cycle
> > to system call code path on my Sandy Bridge CPU.
> 
> Personally, I'm willing to pay that cycle.  It could be a bigger 
> savings on context switch, and the simplification it enables is 
> pretty good.

But, but ... context switches are a relative slow path, compared to 
system calls. And I say this with the scheduler maintainer hat on as 
well.

So this is not a good bargain IMHO, assuming it's not some _huge_ 
difference in maintainability - but having an extra percpu field
isn't really much of a problem.

I don't claim that we couldn't in some other situation decide that a 
certain type of speedup isn't worth it - but what's the big problem 
here? A bit of arithmetics shouldn't be a problem?

Thanks,

	Ingo
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