linux-kernel - Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur

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Date:	Fri, 1 Apr 2016 09:56:36 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Len Brown <lenb@...nel.org>
cc:	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and
 scaling_cur_freqy

On Fri, 1 Apr 2016, Len Brown wrote:
> +/*
> + * aperfmperf_snapshot_khz()
> + * On the current CPU, snapshot APERF, MPERF, and jiffies
> + * unless we already did it within 100ms
> + * calculate kHz, save snapshot
> + */
> +static void aperfmperf_snapshot_khz(void *dummy)
> +{
> +	unsigned long long aperf, aperf_delta;
> +	unsigned long long mperf, mperf_delta;
> +	unsigned long long numerator;
> +	struct aperfmperf_sample *s = &get_cpu_var(samples);

this_cpu_ptr is sufficient. That's a smp function call ...

> +
> +	/* Cache KHz for 100 ms */
> +	if (time_before(jiffies, s->jiffies + HZ/10))
> +		goto out;
> +
> +	rdmsrl(MSR_IA32_APERF, aperf);
> +	rdmsrl(MSR_IA32_MPERF, mperf);
> +
> +	aperf_delta = aperf - s->aperf;
> +	mperf_delta = mperf - s->mperf;
> +
> +	/*
> +	 * There is no architectural guarantee that MPERF
> +	 * increments faster than we can read it.
> +	 */
> +	if (mperf_delta == 0)
> +		goto out;
> +
> +	numerator = cpu_khz * aperf_delta;
> +	s->khz = div64_u64(numerator, mperf_delta);
> +	s->jiffies = jiffies;
> +	s->aperf = aperf;
> +	s->mperf = mperf;
> +
> +out:
> +	put_cpu_var(samples);
> +}
> +
> +unsigned int aperfmperf_khz_on_cpu(int cpu)
> +{
> +	if (!cpu_khz)
> +		return 0;
> +
> +	if (!boot_cpu_has(X86_FEATURE_APERFMPERF))
> +		return 0;
> +
> +	smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);

You can avoid the function call if you check s->jiffies here.

Thanks,

	tglx