Re: Enabling RDPMC in user space by default

Zwane Mwaikambo
2005-12-01T04:02:31Z

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