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Subject[PATCH v2] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive
Date
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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>
---

-> v2:
* Follow the Doug's suggestion and drop the immediate jump to
max P-state if boost is max. The code is simpler this way and
the perf impact should not be noticeable on average.

---
drivers/cpufreq/intel_pstate.c | 36 ++++++++++++++++++------------------
1 file changed, 18 insertions(+), 18 deletions(-)

Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/intel_pstate.c
+++ linux-pm/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)
@@ -1679,17 +1681,14 @@ static inline int32_t get_avg_pstate(str
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;

@@ -1768,29 +1767,30 @@ static void intel_pstate_update_util(str
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);
}
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