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

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wyso...@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 <dsmyth...@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@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);
 }