linux-kernel - Re: [PATCH v2] x86: Reduce clock calibration time during slave cpu startup

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Message-ID: <20110807003642.GA4442@srcf.ucam.org>
Date:	Sun, 7 Aug 2011 01:36:42 +0100
From:	Matthew Garrett <mjg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jack Steiner <steiner@....com>, tglx@...utronix.de,
	davej@...hat.com, yinghan@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [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@...f.ucam.org
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