linux-kernel - Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid scaling issues

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Message-ID: <20150424095906.GL13605@tassilo.jf.intel.com>
Date:	Fri, 24 Apr 2015 02:59:06 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Anton Blanchard <anton@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid
 scaling issues

> There are better ways to do that than using heuristics. We have to
> deal with 3 variants of the reference counter:
> 
> 1) Core and Atom: counts bus cycles and we know that frequency already
>    	    	  from the local apic calibration
> 
> 2) Nehalem, Westmere: Same as TSC
> 
> 3) Sandybridge and later:  XCLK which is 100MHz
> 
> No magic calibration, just use the information which we have on our
> hands already.

This is a really bad idea. We basically would need to maintain a big
switch with model numbers, with new cases added for every new CPU.
Would be a maintenance nightmare.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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