cpufreq: x86: Make scaling_cur_freq behave more as expected

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>

diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
index d869c8671e364d55e343322b410fec9a3e5f66c6..7cf7c70b6ef2a20483361fb8333a85ace25bf1d7 100644 (file)
--- 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);
 }