Re: [UPDATE][PATCH v3 1/2] cpufreq: intel_pstate: Allow enable/disable energy efficiency

[Borislav Petkov]

On Thu, Jun 25, 2020 at 03:49:31PM -0700, Srinivas Pandruvada wrote:
> By default intel_pstate driver disables energy efficiency by setting
> MSR_IA32_POWER_CTL bit 19 for Kaby Lake desktop CPU model in HWP mode.
> This CPU model is also shared by Coffee Lake desktop CPUs. This allows
> these systems to reach maximum possible frequency. But this adds power
> penalty, which some customers don't want. They want some way to enable/
> disable dynamically.
> 
> So, add an additional attribute "energy_efficiency_enable" under
> /sys/devices/system/cpu/intel_pstate/ for these CPU models. This allows
> to read and write bit 19 ("Disable Energy Efficiency Optimization") in
> the MSR IA32_POWER_CTL.

Yes, this is how functionality behind MSRs should be made available to
userspace - not poking at naked MSRs. Good.

> This attribute is present in both HWP and non-HWP mode as this has an
> effect in both modes. Refer to Intel Software Developer's manual for
> details. The scope of this bit is package wide. Also these systems
> support only one package. So read/write MSR on the current CPU is
> enough.
> 
> Suggested-by: Len Brown <lenb@xxxxxxxxxx>
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> ---
> v3 update
> Moved the MSR bit definition to msr-index.h from intel_pstate.c as Doug
> wanted. Offline checking with Borislav, for MSR defintion it is
> fine to move to  msr-index.h even for single user of the definition. But
> here the MSR definition is already in msr-index.h, but adding the MSR bit
> definition also.

Yes.

Btw, no need for the "offline checking" - you can do this on the mailing
list just fine.

>  Documentation/admin-guide/pm/intel_pstate.rst |  9 ++++
>  arch/x86/include/asm/msr-index.h              |  1 +
>  drivers/cpufreq/intel_pstate.c                | 47 ++++++++++++++++++-
>  3 files changed, 55 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/pm/intel_pstate.rst b/Documentation/admin-guide/pm/intel_pstate.rst
> index 39d80bc29ccd..1ca2684a94d7 100644
> --- a/Documentation/admin-guide/pm/intel_pstate.rst
> +++ b/Documentation/admin-guide/pm/intel_pstate.rst
> @@ -431,6 +431,15 @@ argument is passed to the kernel in the command line.
>  	supported in the current configuration, writes to this attribute will
>  	fail with an appropriate error.
>  
> +``energy_efficiency_enable``
> +	This attribute is only present on platforms, which has CPUs matching

						which have

> +	Kaby Lake or Coffee Lake desktop CPU model. By default
> +	"energy_efficiency" is disabled on these CPU models in HWP mode by this
> +	driver. Enabling energy efficiency may limit maximum operating
> +	frequency in both HWP and non HWP mode. In non HWP mode, this attribute
> +	has an effect in turbo range only. But in HWP mode, this attribute also
> +	has an effect in non turbo range.

Those last two sentences could be simplified - read strange.

> +
>  Interpretation of Policy Attributes
>  -----------------------------------
>  
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index e8370e64a155..fec86ad14f8d 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -254,6 +254,7 @@
>  #define MSR_PEBS_FRONTEND		0x000003f7
>  
>  #define MSR_IA32_POWER_CTL		0x000001fc
> +#define MSR_IA32_POWER_CTL_BIT_EE	19

Sort that MSR in - I know, the rest is not sorted either but we can
start somewhere. So pls put it...

#define MSR_LBR_SELECT                  0x000001c8
#define MSR_LBR_TOS                     0x000001c9

<--- here.

#define MSR_LBR_NHM_FROM                0x00000680
#define MSR_LBR_NHM_TO                  0x000006c0


>  #define MSR_IA32_MC0_CTL		0x00000400
>  #define MSR_IA32_MC0_STATUS		0x00000401
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 8e23a698ce04..daa1d9c12098 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -1218,6 +1218,42 @@ static ssize_t store_hwp_dynamic_boost(struct kobject *a,
>  	return count;
>  }
>  
> +static ssize_t show_energy_efficiency_enable(struct kobject *kobj,
> +					     struct kobj_attribute *attr,
> +					     char *buf)
> +{
> +	u64 power_ctl;
> +	int enable;
> +
> +	rdmsrl(MSR_IA32_POWER_CTL, power_ctl);
> +	enable = (power_ctl & BIT(MSR_IA32_POWER_CTL_BIT_EE)) >> MSR_IA32_POWER_CTL_BIT_EE;

So you can simplify to:

	enable = !!(power_ctl & BIT(MSR_IA32_POWER_CTL_BIT_EE));

methinks.

> +	return sprintf(buf, "%d\n", !enable);

If this bit is called

	"Disable Energy Efficiency Optimization"

why do you call your function and sysfs file "enable"? This is making it
more confusing.

Why don't you call it simply: "energy_efficiency" and have it intuitive:

1 - enabled
0 - disabled

?

> +static ssize_t store_energy_efficiency_enable(struct kobject *a,
> +					      struct kobj_attribute *b,
> +					      const char *buf, size_t count)
> +{
> +	u64 power_ctl;
> +	u32 input;
> +	int ret;
> +
> +	ret = kstrtouint(buf, 10, &input);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&intel_pstate_driver_lock);
> +	rdmsrl(MSR_IA32_POWER_CTL, power_ctl);
> +	if (input)

This is too lax - it will be enabled for any !0 value. Please accept
only 0 and 1.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette