Commit 4815d3c5 authored by Rafael J. Wysocki's avatar Rafael J. Wysocki
Browse files

cpufreq: x86: Make scaling_cur_freq behave more as expected 



After commit f8475cef "x86: use common aperfmperf_khz_on_cpu() to
calculate KHz using APERF/MPERF" the scaling_cur_freq policy attribute
in sysfs only behaves as expected on x86 with APERF/MPERF registers
available when it is read from at least twice in a row.  The value
returned by the first read may not be meaningful, because the
computations in there use cached values from the previous iteration
of aperfmperf_snapshot_khz() which may be stale.

To prevent that from happening, modify arch_freq_get_on_cpu() to
call aperfmperf_snapshot_khz() twice, with a short delay between
these calls, if the previous invocation of aperfmperf_snapshot_khz()
was too far back in the past (specifically, more that 1s ago).

Also, as pointed out by Doug Smythies, aperf_delta is limited now
and the multiplication of it by cpu_khz won't overflow, so simplify
the s->khz computations too.

Fixes: f8475cef "x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF"
Reported-by: default avatarDoug Smythies <dsmythies@telus.net>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
parent 520eccdf

arch/x86/kernel/cpu/aperfmperf.c

+26 −14
Original line number Diff line number Diff line
@@ -8,20 +8,25 @@ 
 * This file is licensed under GPLv2.

 */



#include <linux/jiffies.h>

#include <linux/delay.h>

#include <linux/ktime.h>

#include <linux/math64.h>

#include <linux/percpu.h>

#include <linux/smp.h>



struct aperfmperf_sample {

	unsigned int	khz;

	unsigned long	jiffies;

	ktime_t	time;

	u64	aperf;

	u64	mperf;

};



static DEFINE_PER_CPU(struct aperfmperf_sample, samples);



#define APERFMPERF_CACHE_THRESHOLD_MS	10

#define APERFMPERF_REFRESH_DELAY_MS	20

#define APERFMPERF_STALE_THRESHOLD_MS	1000



/*

 * aperfmperf_snapshot_khz()

 * On the current CPU, snapshot APERF, MPERF, and jiffies
@@ -33,9 +38,11 @@ static void aperfmperf_snapshot_khz(void *dummy) 
	u64 aperf, aperf_delta;

	u64 mperf, mperf_delta;

	struct aperfmperf_sample *s = this_cpu_ptr(&samples);

	ktime_t now = ktime_get();

	s64 time_delta = ktime_ms_delta(now, s->time);



	/* Don't bother re-computing within 10 ms */

	if (time_before(jiffies, s->jiffies + HZ/100))

	/* Don't bother re-computing within the cache threshold time. */

	if (time_delta < APERFMPERF_CACHE_THRESHOLD_MS)

		return;



	rdmsrl(MSR_IA32_APERF, aperf);
@@ -51,22 +58,21 @@ static void aperfmperf_snapshot_khz(void *dummy) 
	if (mperf_delta == 0)

		return;



	/*

	 * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then

	 *	khz = (cpu_khz * aperf_delta) / mperf_delta

	 */

	if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta)

		s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);

	else	/* khz = aperf_delta / (mperf_delta / cpu_khz) */

		s->khz = div64_u64(aperf_delta,

			div64_u64(mperf_delta, cpu_khz));

	s->jiffies = jiffies;

	s->time = now;

	s->aperf = aperf;

	s->mperf = mperf;



	/* If the previous iteration was too long ago, discard it. */

	if (time_delta > APERFMPERF_STALE_THRESHOLD_MS)

		s->khz = 0;

	else

		s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);

}



unsigned int arch_freq_get_on_cpu(int cpu)

{

	unsigned int khz;



	if (!cpu_khz)

		return 0;
@@ -74,6 +80,12 @@ unsigned int arch_freq_get_on_cpu(int cpu) 
		return 0;



	smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);

	khz = per_cpu(samples.khz, cpu);

	if (khz)

		return khz;



	msleep(APERFMPERF_REFRESH_DELAY_MS);

	smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);



	return per_cpu(samples.khz, cpu);

}

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