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

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Date:	Mon, 17 May 2010 12:26:35 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"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

> Yes, understood.  But the kernel doesn't expose a "gettimeofday
> performance sucks" flag either.  If it did (or in the case of
> the patch, if tsc_reliable is zero) the application could at least
> choose to turn off the 10000-100000 timestamps/second and log
> a message saying "you are running on old hardware so you get
> fewer features".

I don't think anyone would object to exporting such a flag if 
it's cleanly designed.

Getting the semantics right for that might be somewhat tricky
though. How is "slow" defined?

> A CPU-hotplugable system is a good example of a case where
> the kernel should expose that tsc_reliable is 0.  (I've heard

That would mean that a large class of systems which
are always hotplug capable (even if it's not used) 
would never get fast TSC time.

Wasn't the goal here to be faster? 

> anecdotally that CPU hotplug into a QPI or Hypertransport system
> will have some other interesting challenges, so may require some
> special kernel parameters anyway.)  Even if tsc_reliable were
> only enabled if a "no-cpu_hotplug" kernel parameter is set,
> that is still useful.  And with cores-per-socket (and even
> nodes-per-socket) going up seemingly every day, multi-socket
> systems will likely be an ever smaller percentage of new
> systems.

Still the people running them will expect as good performance
as possible.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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