linux-kernel - Re: [PATCH] init: skip calibration delay if previously done

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Date:	Fri, 3 Jun 2011 14:00:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sameer Nanda <snanda@...omium.org>
Cc:	ext-phil.2.carmody@...ia.com, Tim.Deegan@...rix.com,
	jbeulich@...ell.com, snanda@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] init: skip calibration delay if previously done

On Tue, 24 May 2011 16:19:06 -0700
Sameer Nanda <snanda@...omium.org> wrote:

> For each CPU, do the calibration delay only once. For subsequent calls,
> use the cached per-CPU value of loops_per_jiffy.
> 
> This saves about 200ms of resume time on dual core Intel Atom N5xx based
> systems. This helps bring down the kernel resume time on such systems from
> about 500ms to about 300ms.
> 
> Signed-off-by: Sameer Nanda <snanda@...omium.org>
> ---
>  init/calibrate.c |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
> 
> diff --git a/init/calibrate.c b/init/calibrate.c
> index 76ac919..47d3408 100644
> --- a/init/calibrate.c
> +++ b/init/calibrate.c
> @@ -183,11 +183,18 @@ recalibrate:
>  	return lpj;
>  }
>  
> +DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
> +
>  void __cpuinit calibrate_delay(void)
>  {
>  	static bool printed;
> +	int this_cpu = smp_processor_id();
>  
> -	if (preset_lpj) {
> +	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
> +		loops_per_jiffy = per_cpu(cpu_loops_per_jiffy, this_cpu);
> +		pr_info("Calibrating delay loop (skipped) "
> +				"already calibrated this CPU previously.. ");
> +	} else if (preset_lpj) {
>  		loops_per_jiffy = preset_lpj;
>  		if (!printed)
>  			pr_info("Calibrating delay loop (skipped) "
> @@ -205,6 +212,7 @@ void __cpuinit calibrate_delay(void)
>  			pr_info("Calibrating delay loop... ");
>  		loops_per_jiffy = calibrate_delay_converge();
>  	}
> +	per_cpu(cpu_loops_per_jiffy, this_cpu) = loops_per_jiffy;
>  	if (!printed)
>  		pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
>  			loops_per_jiffy/(500000/HZ),

Seems reasonable.

On resume, the kernel will print "already calibrated this CPU
previously.." in all situations, such as when preset_lpj was set.  I
don't see a problem with that.

Let's be nice to the namespace:

--- a/init/calibrate.c~init-skip-calibration-delay-if-previously-done-fix
+++ a/init/calibrate.c
@@ -246,7 +246,7 @@ recalibrate:
 	return lpj;
 }
 
-DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
+static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
 
 void __cpuinit calibrate_delay(void)
 {
_

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