From febce40febcff3ccdb33f63456ffc4cfc61640c8 Mon Sep 17 00:00:00 2001 From: "Rafael J. Wysocki" Date: Sat, 2 Apr 2016 01:06:21 +0200 Subject: [PATCH] intel_pstate: Avoid extra invocation of intel_pstate_sample() The initialization of intel_pstate for a given CPU involves populating the fields of its struct cpudata that represent the previous sample, but currently that is done in a problematic way. Namely, intel_pstate_init_cpu() makes an extra call to intel_pstate_sample() so it reads the current register values that will be used to populate the "previous sample" record during the next invocation of intel_pstate_sample(). However, after commit a4675fbc4a7a (cpufreq: intel_pstate: Replace timers with utilization update callbacks) that doesn't work for last_sample_time, because the time value is passed to intel_pstate_sample() as an argument now. Passing 0 to it from intel_pstate_init_cpu() is problematic, because that causes cpu->last_sample_time == 0 to be visible in get_target_pstate_use_performance() (and hence the extra cpu->last_sample_time > 0 check in there) and effectively allows the first invocation of intel_pstate_sample() from intel_pstate_update_util() to happen immediately after the initialization which may lead to a significant "turn on" effect in the governor algorithm. To mitigate that issue, rework the initialization to avoid the extra intel_pstate_sample() call from intel_pstate_init_cpu(). Instead, make intel_pstate_sample() return false if it has been called with cpu->sample.time equal to zero, which will make intel_pstate_update_util() skip the sample in that case, and reset cpu->sample.time from intel_pstate_set_update_util_hook() to make the algorithm start properly every time the hook is set. Signed-off-by: Rafael J. Wysocki --- drivers/cpufreq/intel_pstate.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 deletions(-) diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c index 81057e48c4de..9ae159631f52 100644 --- a/drivers/cpufreq/intel_pstate.c +++ b/drivers/cpufreq/intel_pstate.c @@ -910,7 +910,14 @@ static inline bool intel_pstate_sample(struct cpudata *cpu, u64 time) cpu->prev_aperf = aperf; cpu->prev_mperf = mperf; cpu->prev_tsc = tsc; - return true; + /* + * First time this function is invoked in a given cycle, all of the + * previous sample data fields are equal to zero or stale and they must + * be populated with meaningful numbers for things to work, so assume + * that sample.time will always be reset before setting the utilization + * update hook and make the caller skip the sample then. + */ + return !!cpu->last_sample_time; } static inline int32_t get_avg_frequency(struct cpudata *cpu) @@ -984,8 +991,7 @@ static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu) * enough period of time to adjust our busyness. */ duration_ns = cpu->sample.time - cpu->last_sample_time; - if ((s64)duration_ns > pid_params.sample_rate_ns * 3 - && cpu->last_sample_time > 0) { + if ((s64)duration_ns > pid_params.sample_rate_ns * 3) { sample_ratio = div_fp(int_tofp(pid_params.sample_rate_ns), int_tofp(duration_ns)); core_busy = mul_fp(core_busy, sample_ratio); @@ -1100,7 +1106,6 @@ static int intel_pstate_init_cpu(unsigned int cpunum) intel_pstate_get_cpu_pstates(cpu); intel_pstate_busy_pid_reset(cpu); - intel_pstate_sample(cpu, 0); cpu->update_util.func = intel_pstate_update_util; @@ -1121,9 +1126,13 @@ static unsigned int intel_pstate_get(unsigned int cpu_num) return get_avg_frequency(cpu); } -static void intel_pstate_set_update_util_hook(unsigned int cpu) +static void intel_pstate_set_update_util_hook(unsigned int cpu_num) { - cpufreq_set_update_util_data(cpu, &all_cpu_data[cpu]->update_util); + struct cpudata *cpu = all_cpu_data[cpu_num]; + + /* Prevent intel_pstate_update_util() from using stale data. */ + cpu->sample.time = 0; + cpufreq_set_update_util_data(cpu_num, &cpu->update_util); } static void intel_pstate_clear_update_util_hook(unsigned int cpu) -- 2.20.1