[PATCH] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[PATCH] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Rafael J. Wysocki]

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>
---
 drivers/cpufreq/intel_pstate.c |   46 +++++++++++++++++++++++++----------------
 1 file changed, 29 insertions(+), 17 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)
@@ -1678,17 +1680,14 @@ 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;
 
@@ -1767,22 +1766,35 @@ 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);
+				cpu->last_update = time;
+				/*
+				 * The last time the busy was 100% so P-state
+				 * was max anyway, so avoid the overhead of
+				 * computation.
+				 */
+				if (fp_toint(cpu->sample.busy_scaled) == 100)
+					return;
+
+				goto set_pstate;
+			}
+		} 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;

[PATCH v2] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Rafael J. Wysocki]

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);
 }

RE: [PATCH v2] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Doug Smythies]

On 2019.02.07 03:51 Rafael J. Wysocki wrote:

> 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.

Hi Rafael,

Something has broken on my incoming e-mail sorting stuff, and I
missed this one (and some others).

This V2 is not actually what I was proposing. I was O.K. with
the immediate jump, but I didn't want the set_pstate step
by-passed if it was already at max because that would also
by-pass the trace sample, if it was enabled.

Anyway, this V2 seems O.K. to me. I tested it compared to V1
and, as you mentioned, wasn't able to detect any energy consumption
or performance differences.

... Doug

Re: [PATCH v2] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Rafael J. Wysocki]

On Sunday, February 17, 2019 8:25:37 PM CET Doug Smythies wrote:
> On 2019.02.07 03:51 Rafael J. Wysocki wrote:
> 
> > 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.
> 
> Hi Rafael,
> 
> Something has broken on my incoming e-mail sorting stuff, and I
> missed this one (and some others).
> 
> This V2 is not actually what I was proposing. I was O.K. with
> the immediate jump, but I didn't want the set_pstate step
> by-passed if it was already at max because that would also
> by-pass the trace sample, if it was enabled.
> 
> Anyway, this V2 seems O.K. to me. I tested it compared to V1
> and, as you mentioned, wasn't able to detect any energy consumption
> or performance differences.

Thanks for the confirmation!

RE: [PATCH] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Doug Smythies]

On 2019.01.30 16:05 Rafael J. Wysocki wrote:

> 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>
> ---
>  drivers/cpufreq/intel_pstate.c |   46 +++++++++++++++++++++++++----------------
>  1 file changed, 29 insertions(+), 17 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)
> @@ -1678,17 +1680,14 @@ 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;
> 
> @@ -1767,22 +1766,35 @@ 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);
> +				cpu->last_update = time;
> +				/*
> +				 * The last time the busy was 100% so P-state
> +				 * was max anyway, so avoid the overhead of
> +				 * computation.
> +				 */
> +				if (fp_toint(cpu->sample.busy_scaled) == 100)
> +					return;

Hi Rafael,

By exiting here, the trace, if enabled, is also bypassed.
We want the trace sample.
Also, there is a generic:
"If the target ptstate is the same as before, then don't set it"
later on.
Suggest to delete this test and exit condition. (I see that this early exit was done before also.)

> +
> +				goto set_pstate;
> +			}
> +		} 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;

Re: [PATCH] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Rafael J. Wysocki]

On Friday, February 1, 2019 5:54:37 PM CET Doug Smythies wrote:
> On 2019.01.30 16:05 Rafael J. Wysocki wrote:
> 
> > 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>
> > ---
> >  drivers/cpufreq/intel_pstate.c |   46 +++++++++++++++++++++++++----------------
> >  1 file changed, 29 insertions(+), 17 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)
> > @@ -1678,17 +1680,14 @@ 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;
> > 
> > @@ -1767,22 +1766,35 @@ 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);
> > +				cpu->last_update = time;
> > +				/*
> > +				 * The last time the busy was 100% so P-state
> > +				 * was max anyway, so avoid the overhead of
> > +				 * computation.
> > +				 */
> > +				if (fp_toint(cpu->sample.busy_scaled) == 100)
> > +					return;
> 
> Hi Rafael,
> 
> By exiting here, the trace, if enabled, is also bypassed.
> We want the trace sample.

Fair enough, but the return is there regardless of this patch.

Maybe it should be fixed separately?

> Also, there is a generic:
> "If the target ptstate is the same as before, then don't set it"
> later on.
> Suggest to delete this test and exit condition. (I see that this early
> exit was done before also.)

Well, exactly.

It is not unreasonable to boost the frequency right away for an IO-waiter
without waiting for the next sample time IMO.

RE: [PATCH] cpufreq: intel_pstate: Rework iowait boosting to be less aggressive

[Doug Smythies]

On 2019.02.05 04:04 Rafael J. Wysocki wrote:
> On Friday, February 1, 2019 5:54:37 PM CET Doug Smythies wrote:
>> On 2019.01.30 16:05 Rafael J. Wysocki wrote:
>> 
>>> 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>
>>> ---
>>>  drivers/cpufreq/intel_pstate.c |   46 +++++++++++++++++++++++++----------------
>>>  1 file changed, 29 insertions(+), 17 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)
>>> @@ -1678,17 +1680,14 @@ 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;
>>> 
>>> @@ -1767,22 +1766,35 @@ 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);
>>> +				cpu->last_update = time;
>>> +				/*
>>> +				 * The last time the busy was 100% so P-state
>>> +				 * was max anyway, so avoid the overhead of
>>> +				 * computation.
>>> +				 */
>>> +				if (fp_toint(cpu->sample.busy_scaled) == 100)
>>> +					return;
>> 
>> Hi Rafael,
>> 
>> By exiting here, the trace, if enabled, is also bypassed.
>> We want the trace sample.
>
> Fair enough, but the return is there regardless of this patch.
>
> Maybe it should be fixed separately?

O.K.

>> Also, there is a generic:
>> "If the target ptstate is the same as before, then don't set it"
>> later on.
>> Suggest to delete this test and exit condition. (I see that this early
>> exit was done before also.)
>
> Well, exactly.
>
> It is not unreasonable to boost the frequency right away for an IO-waiter
> without waiting for the next sample time IMO.

I agree, but am just saying that it should include a trace sample, otherwise
it is difficult to understand what happened.

By the way, I forgot to mention before, I tried the patch and it does prevent
CPU frequency spikes to maximum every few seconds in a very idle system.

... Doug