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Date: Sat, 13 Aug 2016 20:45:34 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Brian Gerst <brgerst@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to()
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* Brian Gerst <brgerst@gmail.com> wrote:

> On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst <brgerst@gmail.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