linux-kernel - Re: Faster getcpu() and sched

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Message-ID: <20080929145408.GQ25711@one.firstfloor.org>
Date:	Mon, 29 Sep 2008 16:54:08 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	dean gaudet <odo@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Pardo <pardo@...gle.com>,
	linux-kernel@...r.kernel.org, mbligh@...gle.com,
	briangrant@...gle.com, nil@...gle.com, jyasskin@...gle.com
Subject: Re: Faster getcpu() and sched_getcpu()

On Mon, Sep 29, 2008 at 12:27:09AM -0700, dean gaudet wrote:
> Andi Kleen wrote:
> > I'm still not sure why you say no redzone is that expensive? Do you
> > have numbers?  I know it's a few instructions, but it shouldn't 
> > be that expensive.
> >
> >   
> 
> it depends on the processor involved and the kernel config options --
> i.e. if frame pointers are enabled then the stack frame guarantees a
> store operation (push rbp) and on processors which do memops in-order
> this delays the other memops in the vsyscall (i.e. testing the cache or
> executing SIDT).  it was 2 or 3 cycles difference in most cases iirc.

Ok frame pointers are always a performance disasters on some CPUs.
Perhaps they should be just unconditionally disabled for vsyscall.c
and the vdso

-Andi
-- 
ak@...ux.intel.com
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