cpufreq: intel_pstate: Proportional algorithm for Atom

The PID algorithm used by the intel_pstate driver tends to drive
performance to the minimum for workloads with utilization below the
setpoint, which is undesirable, so replace it with a modified
"proportional" algorithm on Atom.

The new algorithm will set the new P-state to be 1.25 times the
available maximum times the (frequency-invariant) utilization during
the previous sampling period except when the target P-state computed
this way is lower than the average P-state during the previous
sampling period.  In the latter case, it will increase the target by
50% of the difference between it and the average P-state to prevent
performance from dropping down too fast in some cases.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>

1 files changed, 21 insertions, 1 deletions

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 1c7b91c5805b..6c8c897d0a2d 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -1232,6 +1232,7 @@ static inline int32_t get_target_pstate_use_cpu_load(struct cpudata *cpu)
 {
         struct sample *sample = &cpu->sample;
         int32_t busy_frac, boost;
+        int target, avg_pstate;
 
         busy_frac = div_fp(sample->mperf, sample->tsc);
 
@@ -1242,7 +1243,26 @@ static inline int32_t get_target_pstate_use_cpu_load(struct cpudata *cpu)
                 busy_frac = boost;
 
         sample->busy_scaled = busy_frac * 100;
-        return get_avg_pstate(cpu) - pid_calc(&cpu->pid, sample->busy_scaled);
+
+        target = limits->no_turbo || limits->turbo_disabled ?
+                        cpu->pstate.max_pstate : cpu->pstate.turbo_pstate;
+        target += target >> 2;
+        target = mul_fp(target, busy_frac);
+        if (target < cpu->pstate.min_pstate)
+                target = cpu->pstate.min_pstate;
+
+        /*
+         * If the average P-state during the previous cycle was higher than the
+         * current target, add 50% of the difference to the target to reduce
+         * possible performance oscillations and offset possible performance
+         * loss related to moving the workload from one CPU to another within
+         * a package/module.
+         */
+        avg_pstate = get_avg_pstate(cpu);
+        if (avg_pstate > target)
+                target += (avg_pstate - target) >> 1;
+
+        return target;
 }
 
 static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu)