linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Disable interrupts during MSRs reading

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Date:	Wed, 30 Apr 2014 00:29:23 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Stratos Karafotis <stratosk@...aphore.gr>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Dirk Brandewie <dirk.j.brandewie@...el.com>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Disable interrupts during MSRs reading

On Tuesday, April 29, 2014 08:46:05 PM Stratos Karafotis wrote:
> According to Intel 64 and IA-32 Architectures SDM, Volume 3,
> Chapter 14.2, "Software needs to exercise care to avoid delays
> between the two RDMSRs (for example interrupts)".
> 
> So, disable interrupts during reading MSRs IA32_APERF and IA32_MPERF.
> Since the TSC is also takes place in the calculation include it in
> the protected section. This should increase the accuracy of the
> calculations.
> 
> Tested on Intel i7-3770 CPU @ 3.40GHz.
> Benchmarks (Phoronix Linux compilation) show a slightly increase
> in performance by ~0.1% and a decrease in power consumption by
> ~0.13%.
> 
> Signed-off-by: Stratos Karafotis <stratosk@...aphore.gr>

This looks good to me, but I need Dirk Brandewie to review and comment
all intel_pstate patches before I apply any of them.

Thanks!

> ---
> 
> I hope it's not too inappropriate to post test results in
> conjunction with my previous patch about the change of calculation
> method of next p-state (https://lkml.org/lkml/2014/4/27/100).
> I used Phoronix benchmark - Linux Kernel Compilation 3.1.
> Below the test results using turbostat (5 iterations):
> 
> Without 2 patches:
> 
> Ph. avg Time	Total time	PkgWatt		Total Energy
> 	79.63	266.416		57.74		15382.85984
> 	79.63	265.609		57.87		15370.79283
> 	79.57	266.994		57.54		15362.83476
> 	79.53	265.304		57.83		15342.53032
> 	79.71	265.977		57.76		15362.83152
> avg	79.61	266.06		57.74		15364.36985
> 
> With change calculation patch:
> 
> Ph. avg Time	Total time	PkgWatt		Total Energy
> 	78.23	258.826		59.14		15306.96964
> 	78.41	259.110		59.15		15326.35650
> 	78.40	258.530		59.26		15320.48780
> 	78.46	258.673		59.20		15313.44160
> 	78.19	259.075		59.16		15326.87700
> avg	78.34	258.842		59.18		15318.82650
> 
> With change calculation patch + this one
> 
> Ph. avg Time	Total time	PkgWatt		Total Energy
> 	78.33	259.320		59.06		15315.43920
> 	78.28	258.245		59.20		15288.10400
> 	78.08	257.523		59.32		15276.26436
> 	78.38	259.084		59.13		15319.63692
> 	78.31	258.232		59.22		15292.49904
> avg	78.27	258.480		59.18		15298.38870
> 
> 
>  drivers/cpufreq/intel_pstate.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 8e309db..95b3958 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -576,10 +576,13 @@ static inline void intel_pstate_sample(struct cpudata *cpu)
>  {
>  	u64 aperf, mperf;
>  	unsigned long long tsc;
> +	unsigned long flags;
>  
> +	local_irq_save(flags);
>  	rdmsrl(MSR_IA32_APERF, aperf);
>  	rdmsrl(MSR_IA32_MPERF, mperf);
>  	tsc = native_read_tsc();
> +	local_irq_restore(flags);
>  
>  	aperf = aperf >> FRAC_BITS;
>  	mperf = mperf >> FRAC_BITS;
> 

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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