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

From: Arjan van de Ven
Date: Sun May 16 2010 - 16:26:58 EST

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

> > From: Thomas Gleixner [mailto:tglx@xxxxxxxxxxxxx]
> > 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

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
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