linux-kernel - Re: [PATCH v3 0/7] x86: Rewrite switch

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Date:	Sat, 13 Aug 2016 20:45:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to()


* Brian Gerst <brgerst@...il.com> wrote:

> On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst <brgerst@...il.com> wrote:
> >> This patch set simplifies the switch_to() code, by moving the stack switch
> >> code out of line into an asm stub before calling __switch_to().  This ends
> >> up being more readable, and using the C calling convention instead of
> >> clobbering all registers improves code generation.  It also allows newly
> >> forked processes to construct a special stack frame to seamlessly flow
> >> to ret_from_fork, instead of using a test and branch, or an unbalanced
> >> call/ret.
> >
> > Do you have performance numbers? Is it noticeable/measurable?
> 
> How do I measure it?  The perf documentation isn't easy to understand.

Something like this:

  taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench sched pipe

... will give a very good idea about the general impact of these changes on 
context switch overhead.

Thanks,

	Ingo