linux-kernel - Re: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <20100516132933.617d1544@infradead.org>
Date:	Sun, 16 May 2010 13:29:33 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	chris.mason@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs

On Sun, 16 May 2010 09:42:40 -0700 (PDT)
Dan Magenheimer <dan.magenheimer@...cle.com> wrote:

> > From: Thomas Gleixner [mailto:tglx@...utronix.de]
> > Nah, there are systems which will have it set to 1:
> > Dig out your good old Pentium-I box and enjoy.
> 
> Hot stove syndrome again?  Are you truly saying that there
> are NO single-socket multi-core systems that don't have
> stupid firmware (SMI and/or BIOS)? 

there are no systems *where we can know* this.
Some of the stupid SMI only triggers on higher temperature situations
etc. Impossible to know upfront.


> If things are this bad, why on earth would the kernel itself
> EVER use TSC even as its own internal clocksource? 

Why do you think we do extensive and continuous validation of the tsc
(and soon, continuous recalibration)


> But that doesn't mean the vast majority of latest generation
> single-socket systems can't set "tsc_reliable" to 1.  Or that
> the kernel is responsible for detecting and/or correcting
> every system with buggy firmware.

sadly this also shows up on single socket systems... much more than we
like.

This is why I really really hate having apps run tsc directly.
A VDSO call at least gives the kernel the option to ensure
correctness... even if it starts out fast and goes slow suddenly after
3 weeks when the AC in the datacenter got maintenance for an hour.





-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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