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Subject[PATCH] cpufreq: x86: Make scaling_cur_freq behave more as expected
Date
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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
aperfmperf_snapshot_khz() call which may be stale. However, the
interface is expected to return meaningful values on every read,
including the first one.

To address this problem 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) and
adjust aperfmperf_snapshot_khz() for that.

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>
---
arch/x86/kernel/cpu/aperfmperf.c | 36 +++++++++++++++++++++++++++++-------
1 file changed, 29 insertions(+), 7 deletions(-)

Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c
===================================================================
--- linux-pm.orig/arch/x86/kernel/cpu/aperfmperf.c
+++ linux-pm/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
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);
@@ -51,6 +58,16 @@ static void aperfmperf_snapshot_khz(void
if (mperf_delta == 0)
return;

+ 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;
+ return;
+ }
+
/*
* if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then
* khz = (cpu_khz * aperf_delta) / mperf_delta
@@ -60,13 +77,12 @@ static void aperfmperf_snapshot_khz(void
else /* khz = aperf_delta / (mperf_delta / cpu_khz) */
s->khz = div64_u64(aperf_delta,
div64_u64(mperf_delta, cpu_khz));
- s->jiffies = jiffies;
- s->aperf = aperf;
- s->mperf = mperf;
}

unsigned int arch_freq_get_on_cpu(int cpu)
{
+ unsigned int khz;
+
if (!cpu_khz)
return 0;

@@ -74,6 +90,12 @@ unsigned int arch_freq_get_on_cpu(int cp
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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