From 4815d3c56d1e10449a44089a47544d9ba84fad0d Mon Sep 17 00:00:00 2001 From: "Rafael J. Wysocki" Date: Fri, 28 Jul 2017 14:45:03 +0200 Subject: [PATCH] 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 Signed-off-by: Rafael J. Wysocki --- 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 +#include +#include #include #include #include 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); } -- 2.34.1