linux-kernel - Re: getting processor numbers

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Date:	Wed, 04 Apr 2007 07:09:32 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: getting processor numbers

H. Peter Anvin a écrit :
> Jeremy Fitzhardinge wrote:
>> H. Peter Anvin wrote:
>>> Sounds like it would need a device which can be waited upon for changes.
>>
>> A vdso-like shared page could have a futex in it.
> 
> Yes, but a futex couldn't be waited upon with a bunch of other things as 
> part of a poll or a select.  The cost of reading the information is 
> minimal.
>

There is one thing that always worried me.

Intel & AMD manuals make clear that mixing data and program in the same page 
is bad for performance.

In particular, x86_64 vsyscall put jiffies and other vsyscall_gtod_data_t 
right in the midle of code. That is certainly not wise.

A probably sane implementation should use two pages, one for code, one for data.

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