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

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Message-ID: <3479976f-5c91-4bcb-8542-3de93bee268a@default>
Date:	Tue, 18 May 2010 12:16:43 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: RE: [PATCH] x86: Export tsc related information in sysfs

> From: Dan Magenheimer
> 
> > From: H. Peter Anvin [mailto:hpa@...or.com]
> > Subject: Re: [PATCH] x86: Export tsc related information in sysfs
> >
> > On 05/18/2010 10:49 AM, Dan Magenheimer wrote:
> > >
> > > (Though I expect tglx/arjan/andi/mingo to disagree with this
> proposal
> > > for similar reasons as the original one that started this
> thread...)
> >
> > I'll add another reason to disagree: exporting these as sysfs
> variables
> > is non-atomic, but these are really only useful when atomically read
> as
> > a unit.
> 
> Which "these"?  The counters?  I would think the primary use
> of the counters is to diagnose extreme problem cases, not
> to differentiate whether a system or process did exactly
> 27 vs 28 rdtsc's, so I don't see why atomic read is at all
> necessary.
> 
> Or did I miss your point entirely?

Oops, hit send too soon.

And the reason I expect tglx/arjan/andi/mingo to disagree is because
their position is that there is NO safe use for rdtsc in userspace EVER!
Whereas your position stated earlier:

> There are restricted uses of the TSC in userspace which are still
> useful
> (mainly involving performance analysis and/or CPU-locked processes).

says there are.

While the engineer in me agrees with tglx/arjan/andi/mingo, the
realist in me agrees with you.
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