Date: Wed, 30 Nov 2005 20:08:08 -0800 (PST)
From: Zwane Mwaikambo <zwane@arm.linux.org.uk>
To: Andi Kleen <ak@suse.de>
cc: discuss@x86-64.org, linux-kernel@vger.kernel.org
Subject: Re: Enabling RDPMC in user space by default
In-Reply-To: <20051130033808.GJ19515@wotan.suse.de>
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On Wed, 30 Nov 2005, Andi Kleen wrote:

> The performance counters definitely share some properties with TSC already -
> they definitely won't be synced (because they don't tick in C states etc.)
> so if you change CPUs they won't be monotone.
> 
> But I doubt we'll ever see them running at a different frequency than
> the current P state, which is the big problem RDTSC has now and that's
> why I'm looking for a replacement. That it's faster on P4 is just
> a bonus.
> 
> However it looks like everybody except me hates the idea :/ Or perhaps
> a lot of the opposition was just against the reasonable position
> that profiling should not disturb the NMI watchdogs. I guess not
> everybody values debuggable kernel dumps.

I agree that the NMI watchdog is very important, but my main objection is 
trying to provide a stable interface for this, i would rather see a 
seperate tool do it (as cumbersome as it may be) even if it meant that 
the external tool simply did what you intend on doing in the kernel.

	Zwane

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