Date: Fri, 11 Mar 2011 15:27:44 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: gregkh@suse.de, linux-kernel@vger.kernel.org, sboyd@codeaurora.org
Subject: Re: [PATCH 3/4] calibrate: retry with wider bounds when converge
 seems to fail
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On Thu, 10 Mar 2011 16:48:06 +0200
Phil Carmody <ext-phil.2.carmody@nokia.com> wrote:

> Systems with unmaskable interrupts such as SMIs may massively underestimate
> loops_per_jiffy, and fail to converge anywhere near the real value. A case
> seen on x86_64 was an initial estimate of 256<<12, which converged to 511<<12
> where the real value should have been over 630<<12. This admitedly requires
> bypassing the TSC calibration (lpj_fine), and a failure to settle in the
> direct calibration too, but is physically possible. This failure does not
> depend on my previous calibration optimisation, but by luck is easy to fix
> with the optimisation in place with a trivial retry loop.
> 
> In the context of the optimised converging method, as we can no longer trust
> the starting estimate, enlarge the search bounds exponentially so that the
> number of retries is logarithmically bounded.
> 
> ...
>
> @@ -165,6 +169,16 @@ static unsigned long __cpuinit calibrate_delay_converge(void)
>  			lpj -= loopadd;
>  		loopadd >>= 1;
>  	}
> +	/*
> +	 * If we incremented every single time possible, presume we've
> +	 * massively underestimated initially, and retry with a higher
> +	 * start, and larger range. (Only seen on x86_64.)
> +	 */
> +	if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
> +		lpj_base = lpj;
> +		loopadd_base <<= 2;
> +		goto recalibrate;
> +	}

This is more informative, no?

--- a/init/calibrate.c~calibrate-retry-with-wider-bounds-when-converge-seems-to-fail-fix
+++ a/init/calibrate.c
@@ -172,7 +172,7 @@ recalibrate:
 	/*
 	 * If we incremented every single time possible, presume we've
 	 * massively underestimated initially, and retry with a higher
-	 * start, and larger range. (Only seen on x86_64.)
+	 * start, and larger range. (Only seen on x86_64, due to SMIs)
 	 */
 	if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
 		lpj_base = lpj;
_

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