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

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Message-ID: <48E0834D.6010608@google.com>
Date:	Mon, 29 Sep 2008 00:27:09 -0700
From:	dean gaudet <odo@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	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()

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.

-dean

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