Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Monday, June 19, 2017 02:28:21 PM Rafael J. Wysocki wrote:
> On Friday, June 16, 2017 09:49:00 PM Len Brown wrote:
> > On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  
> > wrote:
> > > On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> > >> From: Len Brown 
> 
> [cut]
> 
> > >
> > > I wonder if we could change intel_pstate_get() to simply return
> > > aperfmperf_khz_on_cpu(cpu_num)?
> > >
> > > That would allow us to avoid the extra branch here and get rid of the
> > > #ifdef x86 from the header.
> > 
> > The reason I put the hook here is specifically so that the same
> > code would always be called on the x86 architecture,
> > no not matter what cpufreq driver is loaded.
> > 
> > Yes, alternatively, all possible driver.get routines could be updated
> > to call the same routine.  That is acpi-cpufreq.c, intel_pstate.c,
> > others?
> 
> Just acpi-cpufreq.c and intel_pstate.c.
> 
> Moreover, I wouldn't change the behavior on systems using acpi-cpufreq.c,
> because why really?

Actually, having thought a bit more about this, I see why that may be useful.

Also the #ifdef CONFIG_X86 in the header is better than an arch header, at
least for now, so I agree with the approach in your patch.

Thanks,
Rafael

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Monday, June 19, 2017 02:28:21 PM Rafael J. Wysocki wrote:
> On Friday, June 16, 2017 09:49:00 PM Len Brown wrote:
> > On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  
> > wrote:
> > > On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> > >> From: Len Brown 
> 
> [cut]
> 
> > >
> > > I wonder if we could change intel_pstate_get() to simply return
> > > aperfmperf_khz_on_cpu(cpu_num)?
> > >
> > > That would allow us to avoid the extra branch here and get rid of the
> > > #ifdef x86 from the header.
> > 
> > The reason I put the hook here is specifically so that the same
> > code would always be called on the x86 architecture,
> > no not matter what cpufreq driver is loaded.
> > 
> > Yes, alternatively, all possible driver.get routines could be updated
> > to call the same routine.  That is acpi-cpufreq.c, intel_pstate.c,
> > others?
> 
> Just acpi-cpufreq.c and intel_pstate.c.
> 
> Moreover, I wouldn't change the behavior on systems using acpi-cpufreq.c,
> because why really?

Actually, having thought a bit more about this, I see why that may be useful.

Also the #ifdef CONFIG_X86 in the header is better than an arch header, at
least for now, so I agree with the approach in your patch.

Thanks,
Rafael

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Friday, June 16, 2017 09:49:00 PM Len Brown wrote:
> On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  wrote:
> > On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> >> From: Len Brown 

[cut]

> >
> > I wonder if we could change intel_pstate_get() to simply return
> > aperfmperf_khz_on_cpu(cpu_num)?
> >
> > That would allow us to avoid the extra branch here and get rid of the
> > #ifdef x86 from the header.
> 
> The reason I put the hook here is specifically so that the same
> code would always be called on the x86 architecture,
> no not matter what cpufreq driver is loaded.
> 
> Yes, alternatively, all possible driver.get routines could be updated
> to call the same routine.  That is acpi-cpufreq.c, intel_pstate.c,
> others?

Just acpi-cpufreq.c and intel_pstate.c.

Moreover, I wouldn't change the behavior on systems using acpi-cpufreq.c,
because why really?

And some users may complain that now what they see in cpuinfo_cur_freq is
different from what they saw there before.

That leaves us with just intel_pstate.

In addition to that, it would be good to avoid reading APERF and MPERF in the
cases when we already have the values.  I have an idea on how to do that, but
I'll just need to prepare a patch.

> Do I think that saving a branch is meaningful in a system call to retrieve
> the average frequency from the kernel?  No, I don't.
>
> I'm not religious about "if (CONFIG_X86)" appearing in cpufreq.h.
> If somebody really cares, then the purist approach would be
> to add an additional arch-specific-hook header -- though that seemed
> overkill to me...

I'm not a fan of arch stuff in general code, C or headers.

If it is necessary to add an arch header, let's add it, but at this point I'm
not sure about that even.

Thanks,
Rafael

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Friday, June 16, 2017 09:49:00 PM Len Brown wrote:
> On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  wrote:
> > On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> >> From: Len Brown 

[cut]

> >
> > I wonder if we could change intel_pstate_get() to simply return
> > aperfmperf_khz_on_cpu(cpu_num)?
> >
> > That would allow us to avoid the extra branch here and get rid of the
> > #ifdef x86 from the header.
> 
> The reason I put the hook here is specifically so that the same
> code would always be called on the x86 architecture,
> no not matter what cpufreq driver is loaded.
> 
> Yes, alternatively, all possible driver.get routines could be updated
> to call the same routine.  That is acpi-cpufreq.c, intel_pstate.c,
> others?

Just acpi-cpufreq.c and intel_pstate.c.

Moreover, I wouldn't change the behavior on systems using acpi-cpufreq.c,
because why really?

And some users may complain that now what they see in cpuinfo_cur_freq is
different from what they saw there before.

That leaves us with just intel_pstate.

In addition to that, it would be good to avoid reading APERF and MPERF in the
cases when we already have the values.  I have an idea on how to do that, but
I'll just need to prepare a patch.

> Do I think that saving a branch is meaningful in a system call to retrieve
> the average frequency from the kernel?  No, I don't.
>
> I'm not religious about "if (CONFIG_X86)" appearing in cpufreq.h.
> If somebody really cares, then the purist approach would be
> to add an additional arch-specific-hook header -- though that seemed
> overkill to me...

I'm not a fan of arch stuff in general code, C or headers.

If it is necessary to add an arch header, let's add it, but at this point I'm
not sure about that even.

Thanks,
Rafael

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Len Brown]

On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  wrote:
> On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
>> From: Len Brown 
>>
>> The goal of this change is to give users a uniform and meaningful
>> result when they read /sys/...cpufreq/scaling_cur_freq
>> on modern x86 hardware, as compared to what they get today.
>>
>> Modern x86 processors include the hardware needed
>> to accurately calculate frequency over an interval --
>> APERF, MPERF, and the TSC.
>>
>> Here we provide an x86 routine to make this calculation
>> on supported hardware, and use it in preference to any
>> driver driver-specific cpufreq_driver.get() routine.
>>
>> MHz is computed like so:
>>
>> MHz = base_MHz * delta_APERF / delta_MPERF
>>
>> MHz is the average frequency of the busy processor
>> over a measurement interval.  The interval is
>> defined to be the time between successive invocations
>> of aperfmperf_khz_on_cpu(), which are expected to to
>> happen on-demand when users read sysfs attribute
>> cpufreq/scaling_cur_freq.
>>
>> As with previous methods of calculating MHz,
>> idle time is excluded.
>>
>> base_MHz above is from TSC calibration global "cpu_khz".
>>
>> This x86 native method to calculate MHz returns a meaningful result
>> no matter if P-states are controlled by hardware or firmware
>> and/or if the Linux cpufreq sub-system is or is-not installed.
>>
>> When this routine is invoked more frequently, the measurement
>> interval becomes shorter.  However, the code limits re-computation
>> to 10ms intervals so that average frequency remains meaningful.
>>
>> Discerning users are encouraged to take advantage of
>> the turbostat(8) utility, which can gracefully handle
>> concurrent measurement intervals of arbitrary length.
>>
>> Signed-off-by: Len Brown 
>> ---
>>  arch/x86/kernel/cpu/Makefile |  1 +
>>  arch/x86/kernel/cpu/aperfmperf.c | 82 
>> 
>>  drivers/cpufreq/cpufreq.c|  7 +++-
>>  include/linux/cpufreq.h  | 13 +++
>>  4 files changed, 102 insertions(+), 1 deletion(-)
>>  create mode 100644 arch/x86/kernel/cpu/aperfmperf.c
>>
>> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
>> index 521..cdf8249 100644
>> --- a/arch/x86/kernel/cpu/Makefile
>> +++ b/arch/x86/kernel/cpu/Makefile
>> @@ -21,6 +21,7 @@ obj-y   += common.o
>>  obj-y+= rdrand.o
>>  obj-y+= match.o
>>  obj-y+= bugs.o
>> +obj-$(CONFIG_CPU_FREQ)   += aperfmperf.o
>>
>>  obj-$(CONFIG_PROC_FS)+= proc.o
>>  obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
>> diff --git a/arch/x86/kernel/cpu/aperfmperf.c 
>> b/arch/x86/kernel/cpu/aperfmperf.c
>> new file mode 100644
>> index 000..5ccf63a
>> --- /dev/null
>> +++ b/arch/x86/kernel/cpu/aperfmperf.c
>> @@ -0,0 +1,82 @@
>> +/*
>> + * x86 APERF/MPERF KHz calculation for
>> + * /sys/.../cpufreq/scaling_cur_freq
>> + *
>> + * Copyright (C) 2017 Intel Corp.
>> + * Author: Len Brown 
>> + *
>> + * This file is licensed under GPLv2.
>> + */
>> +
>> +#include 
>> +#include 
>> +#include 
>> +#include 
>> +
>> +struct aperfmperf_sample {
>> + unsigned int khz;
>> + unsigned long jiffies;
>> + u64 aperf;
>> + u64 mperf;
>> +};
>> +
>> +static DEFINE_PER_CPU(struct aperfmperf_sample, samples);
>> +
>> +/*
>> + * aperfmperf_snapshot_khz()
>> + * On the current CPU, snapshot APERF, MPERF, and jiffies
>> + * unless we already did it within 10ms
>> + * calculate kHz, save snapshot
>> + */
>> +static void aperfmperf_snapshot_khz(void *dummy)
>> +{
>> + u64 aperf, aperf_delta;
>> + u64 mperf, mperf_delta;
>> + struct aperfmperf_sample *s = _cpu_var(samples);
>> +
>> + /* Don't bother re-computing within 10 ms */
>> + if (time_before(jiffies, s->jiffies + HZ/100))
>> + goto out;
>> +
>> + rdmsrl(MSR_IA32_APERF, aperf);
>> + rdmsrl(MSR_IA32_MPERF, mperf);
>> +
>> + aperf_delta = aperf - s->aperf;
>> + mperf_delta = mperf - s->mperf;
>> +
>> + /*
>> +  * There is no architectural guarantee that MPERF
>> +  * increments faster than we can read it.
>> +  */
>> + if (mperf_delta == 0)
>> + goto out;
>> +
>> + /*
>> +  * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then
>> +  *  khz = (cpu_khz * aperf_delta) / mperf_delta
>> +  */
>> + if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta)
>> + s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
>> + else/* khz = aperf_delta / (mperf_delta / cpu_khz) */
>> + s->khz = div64_u64(aperf_delta,
>> + div64_u64(mperf_delta, cpu_khz));
>> + s->jiffies = jiffies;
>> + s->aperf = aperf;
>> + s->mperf = mperf;
>> +
>> +out:
>> + put_cpu_var(samples);
>> +}
>> +
>> +unsigned int 

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Len Brown]

On Fri, Jun 16, 2017 at 8:30 PM, Rafael J. Wysocki  wrote:
> On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
>> From: Len Brown 
>>
>> The goal of this change is to give users a uniform and meaningful
>> result when they read /sys/...cpufreq/scaling_cur_freq
>> on modern x86 hardware, as compared to what they get today.
>>
>> Modern x86 processors include the hardware needed
>> to accurately calculate frequency over an interval --
>> APERF, MPERF, and the TSC.
>>
>> Here we provide an x86 routine to make this calculation
>> on supported hardware, and use it in preference to any
>> driver driver-specific cpufreq_driver.get() routine.
>>
>> MHz is computed like so:
>>
>> MHz = base_MHz * delta_APERF / delta_MPERF
>>
>> MHz is the average frequency of the busy processor
>> over a measurement interval.  The interval is
>> defined to be the time between successive invocations
>> of aperfmperf_khz_on_cpu(), which are expected to to
>> happen on-demand when users read sysfs attribute
>> cpufreq/scaling_cur_freq.
>>
>> As with previous methods of calculating MHz,
>> idle time is excluded.
>>
>> base_MHz above is from TSC calibration global "cpu_khz".
>>
>> This x86 native method to calculate MHz returns a meaningful result
>> no matter if P-states are controlled by hardware or firmware
>> and/or if the Linux cpufreq sub-system is or is-not installed.
>>
>> When this routine is invoked more frequently, the measurement
>> interval becomes shorter.  However, the code limits re-computation
>> to 10ms intervals so that average frequency remains meaningful.
>>
>> Discerning users are encouraged to take advantage of
>> the turbostat(8) utility, which can gracefully handle
>> concurrent measurement intervals of arbitrary length.
>>
>> Signed-off-by: Len Brown 
>> ---
>>  arch/x86/kernel/cpu/Makefile |  1 +
>>  arch/x86/kernel/cpu/aperfmperf.c | 82 
>> 
>>  drivers/cpufreq/cpufreq.c|  7 +++-
>>  include/linux/cpufreq.h  | 13 +++
>>  4 files changed, 102 insertions(+), 1 deletion(-)
>>  create mode 100644 arch/x86/kernel/cpu/aperfmperf.c
>>
>> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
>> index 521..cdf8249 100644
>> --- a/arch/x86/kernel/cpu/Makefile
>> +++ b/arch/x86/kernel/cpu/Makefile
>> @@ -21,6 +21,7 @@ obj-y   += common.o
>>  obj-y+= rdrand.o
>>  obj-y+= match.o
>>  obj-y+= bugs.o
>> +obj-$(CONFIG_CPU_FREQ)   += aperfmperf.o
>>
>>  obj-$(CONFIG_PROC_FS)+= proc.o
>>  obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
>> diff --git a/arch/x86/kernel/cpu/aperfmperf.c 
>> b/arch/x86/kernel/cpu/aperfmperf.c
>> new file mode 100644
>> index 000..5ccf63a
>> --- /dev/null
>> +++ b/arch/x86/kernel/cpu/aperfmperf.c
>> @@ -0,0 +1,82 @@
>> +/*
>> + * x86 APERF/MPERF KHz calculation for
>> + * /sys/.../cpufreq/scaling_cur_freq
>> + *
>> + * Copyright (C) 2017 Intel Corp.
>> + * Author: Len Brown 
>> + *
>> + * This file is licensed under GPLv2.
>> + */
>> +
>> +#include 
>> +#include 
>> +#include 
>> +#include 
>> +
>> +struct aperfmperf_sample {
>> + unsigned int khz;
>> + unsigned long jiffies;
>> + u64 aperf;
>> + u64 mperf;
>> +};
>> +
>> +static DEFINE_PER_CPU(struct aperfmperf_sample, samples);
>> +
>> +/*
>> + * aperfmperf_snapshot_khz()
>> + * On the current CPU, snapshot APERF, MPERF, and jiffies
>> + * unless we already did it within 10ms
>> + * calculate kHz, save snapshot
>> + */
>> +static void aperfmperf_snapshot_khz(void *dummy)
>> +{
>> + u64 aperf, aperf_delta;
>> + u64 mperf, mperf_delta;
>> + struct aperfmperf_sample *s = _cpu_var(samples);
>> +
>> + /* Don't bother re-computing within 10 ms */
>> + if (time_before(jiffies, s->jiffies + HZ/100))
>> + goto out;
>> +
>> + rdmsrl(MSR_IA32_APERF, aperf);
>> + rdmsrl(MSR_IA32_MPERF, mperf);
>> +
>> + aperf_delta = aperf - s->aperf;
>> + mperf_delta = mperf - s->mperf;
>> +
>> + /*
>> +  * There is no architectural guarantee that MPERF
>> +  * increments faster than we can read it.
>> +  */
>> + if (mperf_delta == 0)
>> + goto out;
>> +
>> + /*
>> +  * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then
>> +  *  khz = (cpu_khz * aperf_delta) / mperf_delta
>> +  */
>> + if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta)
>> + s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
>> + else/* khz = aperf_delta / (mperf_delta / cpu_khz) */
>> + s->khz = div64_u64(aperf_delta,
>> + div64_u64(mperf_delta, cpu_khz));
>> + s->jiffies = jiffies;
>> + s->aperf = aperf;
>> + s->mperf = mperf;
>> +
>> +out:
>> + put_cpu_var(samples);
>> +}
>> +
>> +unsigned int aperfmperf_khz_on_cpu(unsigned int cpu)
>> +{
>> + if (!cpu_khz)
>> + return 0;
>> +
>> + 

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> From: Len Brown 
> 
> The goal of this change is to give users a uniform and meaningful
> result when they read /sys/...cpufreq/scaling_cur_freq
> on modern x86 hardware, as compared to what they get today.
> 
> Modern x86 processors include the hardware needed
> to accurately calculate frequency over an interval --
> APERF, MPERF, and the TSC.
> 
> Here we provide an x86 routine to make this calculation
> on supported hardware, and use it in preference to any
> driver driver-specific cpufreq_driver.get() routine.
> 
> MHz is computed like so:
> 
> MHz = base_MHz * delta_APERF / delta_MPERF
> 
> MHz is the average frequency of the busy processor
> over a measurement interval.  The interval is
> defined to be the time between successive invocations
> of aperfmperf_khz_on_cpu(), which are expected to to
> happen on-demand when users read sysfs attribute
> cpufreq/scaling_cur_freq.
> 
> As with previous methods of calculating MHz,
> idle time is excluded.
> 
> base_MHz above is from TSC calibration global "cpu_khz".
> 
> This x86 native method to calculate MHz returns a meaningful result
> no matter if P-states are controlled by hardware or firmware
> and/or if the Linux cpufreq sub-system is or is-not installed.
> 
> When this routine is invoked more frequently, the measurement
> interval becomes shorter.  However, the code limits re-computation
> to 10ms intervals so that average frequency remains meaningful.
> 
> Discerning users are encouraged to take advantage of
> the turbostat(8) utility, which can gracefully handle
> concurrent measurement intervals of arbitrary length.
> 
> Signed-off-by: Len Brown 
> ---
>  arch/x86/kernel/cpu/Makefile |  1 +
>  arch/x86/kernel/cpu/aperfmperf.c | 82 
> 
>  drivers/cpufreq/cpufreq.c|  7 +++-
>  include/linux/cpufreq.h  | 13 +++
>  4 files changed, 102 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/kernel/cpu/aperfmperf.c
> 
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 521..cdf8249 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -21,6 +21,7 @@ obj-y   += common.o
>  obj-y+= rdrand.o
>  obj-y+= match.o
>  obj-y+= bugs.o
> +obj-$(CONFIG_CPU_FREQ)   += aperfmperf.o
>  
>  obj-$(CONFIG_PROC_FS)+= proc.o
>  obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
> diff --git a/arch/x86/kernel/cpu/aperfmperf.c 
> b/arch/x86/kernel/cpu/aperfmperf.c
> new file mode 100644
> index 000..5ccf63a
> --- /dev/null
> +++ b/arch/x86/kernel/cpu/aperfmperf.c
> @@ -0,0 +1,82 @@
> +/*
> + * x86 APERF/MPERF KHz calculation for
> + * /sys/.../cpufreq/scaling_cur_freq
> + *
> + * Copyright (C) 2017 Intel Corp.
> + * Author: Len Brown 
> + *
> + * This file is licensed under GPLv2.
> + */
> +
> +#include 
> +#include 
> +#include 
> +#include 
> +
> +struct aperfmperf_sample {
> + unsigned int khz;
> + unsigned long jiffies;
> + u64 aperf;
> + u64 mperf;
> +};
> +
> +static DEFINE_PER_CPU(struct aperfmperf_sample, samples);
> +
> +/*
> + * aperfmperf_snapshot_khz()
> + * On the current CPU, snapshot APERF, MPERF, and jiffies
> + * unless we already did it within 10ms
> + * calculate kHz, save snapshot
> + */
> +static void aperfmperf_snapshot_khz(void *dummy)
> +{
> + u64 aperf, aperf_delta;
> + u64 mperf, mperf_delta;
> + struct aperfmperf_sample *s = _cpu_var(samples);
> +
> + /* Don't bother re-computing within 10 ms */
> + if (time_before(jiffies, s->jiffies + HZ/100))
> + goto out;
> +
> + rdmsrl(MSR_IA32_APERF, aperf);
> + rdmsrl(MSR_IA32_MPERF, mperf);
> +
> + aperf_delta = aperf - s->aperf;
> + mperf_delta = mperf - s->mperf;
> +
> + /*
> +  * There is no architectural guarantee that MPERF
> +  * increments faster than we can read it.
> +  */
> + if (mperf_delta == 0)
> + goto out;
> +
> + /*
> +  * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then
> +  *  khz = (cpu_khz * aperf_delta) / mperf_delta
> +  */
> + if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta)
> + s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
> + else/* khz = aperf_delta / (mperf_delta / cpu_khz) */
> + s->khz = div64_u64(aperf_delta,
> + div64_u64(mperf_delta, cpu_khz));
> + s->jiffies = jiffies;
> + s->aperf = aperf;
> + s->mperf = mperf;
> +
> +out:
> + put_cpu_var(samples);
> +}
> +
> +unsigned int aperfmperf_khz_on_cpu(unsigned int cpu)
> +{
> + if (!cpu_khz)
> + return 0;
> +
> + if (!static_cpu_has(X86_FEATURE_APERFMPERF))
> + return 0;
> +
> + smp_call_function_single(cpu, 

Re: [PATCH 2/5] x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF

[Rafael J. Wysocki]

On Wednesday, June 07, 2017 07:39:13 PM Len Brown wrote:
> From: Len Brown 
> 
> The goal of this change is to give users a uniform and meaningful
> result when they read /sys/...cpufreq/scaling_cur_freq
> on modern x86 hardware, as compared to what they get today.
> 
> Modern x86 processors include the hardware needed
> to accurately calculate frequency over an interval --
> APERF, MPERF, and the TSC.
> 
> Here we provide an x86 routine to make this calculation
> on supported hardware, and use it in preference to any
> driver driver-specific cpufreq_driver.get() routine.
> 
> MHz is computed like so:
> 
> MHz = base_MHz * delta_APERF / delta_MPERF
> 
> MHz is the average frequency of the busy processor
> over a measurement interval.  The interval is
> defined to be the time between successive invocations
> of aperfmperf_khz_on_cpu(), which are expected to to
> happen on-demand when users read sysfs attribute
> cpufreq/scaling_cur_freq.
> 
> As with previous methods of calculating MHz,
> idle time is excluded.
> 
> base_MHz above is from TSC calibration global "cpu_khz".
> 
> This x86 native method to calculate MHz returns a meaningful result
> no matter if P-states are controlled by hardware or firmware
> and/or if the Linux cpufreq sub-system is or is-not installed.
> 
> When this routine is invoked more frequently, the measurement
> interval becomes shorter.  However, the code limits re-computation
> to 10ms intervals so that average frequency remains meaningful.
> 
> Discerning users are encouraged to take advantage of
> the turbostat(8) utility, which can gracefully handle
> concurrent measurement intervals of arbitrary length.
> 
> Signed-off-by: Len Brown 
> ---
>  arch/x86/kernel/cpu/Makefile |  1 +
>  arch/x86/kernel/cpu/aperfmperf.c | 82 
> 
>  drivers/cpufreq/cpufreq.c|  7 +++-
>  include/linux/cpufreq.h  | 13 +++
>  4 files changed, 102 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/kernel/cpu/aperfmperf.c
> 
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 521..cdf8249 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -21,6 +21,7 @@ obj-y   += common.o
>  obj-y+= rdrand.o
>  obj-y+= match.o
>  obj-y+= bugs.o
> +obj-$(CONFIG_CPU_FREQ)   += aperfmperf.o
>  
>  obj-$(CONFIG_PROC_FS)+= proc.o
>  obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
> diff --git a/arch/x86/kernel/cpu/aperfmperf.c 
> b/arch/x86/kernel/cpu/aperfmperf.c
> new file mode 100644
> index 000..5ccf63a
> --- /dev/null
> +++ b/arch/x86/kernel/cpu/aperfmperf.c
> @@ -0,0 +1,82 @@
> +/*
> + * x86 APERF/MPERF KHz calculation for
> + * /sys/.../cpufreq/scaling_cur_freq
> + *
> + * Copyright (C) 2017 Intel Corp.
> + * Author: Len Brown 
> + *
> + * This file is licensed under GPLv2.
> + */
> +
> +#include 
> +#include 
> +#include 
> +#include 
> +
> +struct aperfmperf_sample {
> + unsigned int khz;
> + unsigned long jiffies;
> + u64 aperf;
> + u64 mperf;
> +};
> +
> +static DEFINE_PER_CPU(struct aperfmperf_sample, samples);
> +
> +/*
> + * aperfmperf_snapshot_khz()
> + * On the current CPU, snapshot APERF, MPERF, and jiffies
> + * unless we already did it within 10ms
> + * calculate kHz, save snapshot
> + */
> +static void aperfmperf_snapshot_khz(void *dummy)
> +{
> + u64 aperf, aperf_delta;
> + u64 mperf, mperf_delta;
> + struct aperfmperf_sample *s = _cpu_var(samples);
> +
> + /* Don't bother re-computing within 10 ms */
> + if (time_before(jiffies, s->jiffies + HZ/100))
> + goto out;
> +
> + rdmsrl(MSR_IA32_APERF, aperf);
> + rdmsrl(MSR_IA32_MPERF, mperf);
> +
> + aperf_delta = aperf - s->aperf;
> + mperf_delta = mperf - s->mperf;
> +
> + /*
> +  * There is no architectural guarantee that MPERF
> +  * increments faster than we can read it.
> +  */
> + if (mperf_delta == 0)
> + goto out;
> +
> + /*
> +  * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then
> +  *  khz = (cpu_khz * aperf_delta) / mperf_delta
> +  */
> + if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta)
> + s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
> + else/* khz = aperf_delta / (mperf_delta / cpu_khz) */
> + s->khz = div64_u64(aperf_delta,
> + div64_u64(mperf_delta, cpu_khz));
> + s->jiffies = jiffies;
> + s->aperf = aperf;
> + s->mperf = mperf;
> +
> +out:
> + put_cpu_var(samples);
> +}
> +
> +unsigned int aperfmperf_khz_on_cpu(unsigned int cpu)
> +{
> + if (!cpu_khz)
> + return 0;
> +
> + if (!static_cpu_has(X86_FEATURE_APERFMPERF))
> + return 0;
> +
> + smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);
> +
> + return per_cpu(samples.khz, cpu);