From c3a14b0ca1837d870e8eca18cedcc1db15a36302 Mon Sep 17 00:00:00 2001

From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>

Date: Fri, 28 Jul 2017 14:45:03 +0200

Subject: [PATCH] cpufreq: x86: Make scaling_cur_freq behave more as expected

References: bnc#1073583 cpufreq reporting

Patch-mainline: v4.13

Git-commit: 4815d3c56d1e10449a44089a47544d9ba84fad0d

After commit f8475cef9008 "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: f8475cef9008 "x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF"

Reported-by: Doug Smythies <dsmythies@telus.net>

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Signed-off-by: Mel Gorman <mgorman@suse.de>

---

 arch/x86/kernel/cpu/aperfmperf.c | 40 ++++++++++++++++++++++++++--------------

 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c

index d869c8671e36..7cf7c70b6ef2 100644

--- a/arch/x86/kernel/cpu/aperfmperf.c

+++ b/arch/x86/kernel/cpu/aperfmperf.c

@@ -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);

 }