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SubjectRe: [PATCH v2] x86: Reduce clock calibration time during slave cpu startup
On Fri, Aug 05, 2011 at 11:38:36PM +0200, Ingo Molnar wrote:

> Well, it still uses heuristics: it assumes frequency is the same when
> the cpuid data tells us that two CPUs are on the same socket, right?

If we only assume that when we have a constant TSC then it's a pretty
safe assumption - the delay loop will be calibrated against the TSC, and
the TSC will be constant across the package regardless of what frequency
the cores are actually running at.

> Cannot we directly see the frequency(ies) of the CPU, and decide
> based on that whether the calibration result can be cached?

Kind of? We can look at the aperf/mperf ratio and compare that against
the TSC to work out the frequency. But that only gives you meaningful
results with a constant TSC, so since you're still depending on that you
might as well skip the extra check.

The only time this should give problems is if someone produces a
multi-core x86 with per-core (rather than per-package) TSC and runs the
TSC at different frequencies per-core. Since that'd probably cost more
than not doing it, I think we're probably safe.

--
Matthew Garrett | mjg59@srcf.ucam.org

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