cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

The current iowait boosting mechanism in intel_pstate_update_util()
is quite aggressive, as it goes to the maximum P-state right away,
and may cause excessive amounts of energy to be used, which is not
desirable and arguably isn't necessary too.

Follow commit a5a0809bc58e ("cpufreq: schedutil: Make iowait boost
more energy efficient") that reworked the analogous iowait boost
mechanism in the schedutil governor and make the iowait boosting
in intel_pstate_update_util() work along the same lines.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 9a6cb3a1be50bbf50e6e66572e9a86c55248d670..002f5169d4ebca567e2753b6f3a7bd509b298c2d 100644 (file)
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -50,6 +50,8 @@
 #define int_tofp(X) ((int64_t)(X) << FRAC_BITS)
 #define fp_toint(X) ((X) >> FRAC_BITS)
 
+#define ONE_EIGHTH_FP ((int64_t)1 << (FRAC_BITS - 3))
+
 #define EXT_BITS 6
 #define EXT_FRAC_BITS (EXT_BITS + FRAC_BITS)
 #define fp_ext_toint(X) ((X) >> EXT_FRAC_BITS)
@@ -1671,17 +1673,14 @@ static inline int32_t get_avg_pstate(struct cpudata *cpu)
 static inline int32_t get_target_pstate(struct cpudata *cpu)
 {
        struct sample *sample = &cpu->sample;
-       int32_t busy_frac, boost;
+       int32_t busy_frac;
        int target, avg_pstate;
 
        busy_frac = div_fp(sample->mperf << cpu->aperf_mperf_shift,
                           sample->tsc);
 
-       boost = cpu->iowait_boost;
-       cpu->iowait_boost >>= 1;
-
-       if (busy_frac < boost)
-               busy_frac = boost;
+       if (busy_frac < cpu->iowait_boost)
+               busy_frac = cpu->iowait_boost;
 
        sample->busy_scaled = busy_frac * 100;
 
@@ -1758,29 +1757,30 @@ static void intel_pstate_update_util(struct update_util_data *data, u64 time,
        if (smp_processor_id() != cpu->cpu)
                return;
 
+       delta_ns = time - cpu->last_update;
        if (flags & SCHED_CPUFREQ_IOWAIT) {
-               cpu->iowait_boost = int_tofp(1);
-               cpu->last_update = time;
-               /*
-                * The last time the busy was 100% so P-state was max anyway
-                * so avoid overhead of computation.
-                */
-               if (fp_toint(cpu->sample.busy_scaled) == 100)
-                       return;
-
-               goto set_pstate;
+               /* Start over if the CPU may have been idle. */
+               if (delta_ns > TICK_NSEC) {
+                       cpu->iowait_boost = ONE_EIGHTH_FP;
+               } else if (cpu->iowait_boost) {
+                       cpu->iowait_boost <<= 1;
+                       if (cpu->iowait_boost > int_tofp(1))
+                               cpu->iowait_boost = int_tofp(1);
+               } else {
+                       cpu->iowait_boost = ONE_EIGHTH_FP;
+               }
        } else if (cpu->iowait_boost) {
                /* Clear iowait_boost if the CPU may have been idle. */
-               delta_ns = time - cpu->last_update;
                if (delta_ns > TICK_NSEC)
                        cpu->iowait_boost = 0;
+               else
+                       cpu->iowait_boost >>= 1;
        }
        cpu->last_update = time;
        delta_ns = time - cpu->sample.time;
        if ((s64)delta_ns < INTEL_PSTATE_SAMPLING_INTERVAL)
                return;
 
-set_pstate:
        if (intel_pstate_sample(cpu, time))
                intel_pstate_adjust_pstate(cpu);
 }