Re: [PATCH v2 1/3] Documentation: admin-guide: pm: Add efficiency vs. latency tradeoff to uncore documentation

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Wed, 28 Aug 2024, Tero Kristo wrote:

> Added documentation about the functionality of efficiency vs. latency tradeoff
> control in intel Xeon processors, and how this is configured via sysfs.
> 
> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
> ---
> v2:
>   * Largely re-wrote the documentation
> 
>  .../pm/intel_uncore_frequency_scaling.rst     | 59 +++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst b/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
> index 5ab3440e6cee..26ded32b06f5 100644
> --- a/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
> +++ b/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
> @@ -113,3 +113,62 @@ to apply at each uncore* level.
>  
>  Support for "current_freq_khz" is available only at each fabric cluster
>  level (i.e., in uncore* directory).
> +
> +Efficiency vs. Latency Tradeoff
> +-------------------------------
> +
> +The Efficiency Latency Control (ELC) feature improves performance
> +per watt. With this feature hardware power management algorithms
> +optimize trade-off between latency and power consumption. For some
> +latency sensitive workloads further tuning can be done by SW to
> +get desired performance.
> +
> +The hardware monitors the average CPU utilization across all cores
> +in a power domain at regular intervals and decides an uncore frequency.
> +While this may result in the best performance per watt, workload may be
> +expecting higher performance at the expense of power. Consider an
> +application that intermittently wakes up to perform memory reads on an
> +otherwise idle system. In such cases, if hardware lowers uncore
> +frequency, then there may be delay in ramp up of frequency to meet
> +target performance.
> +
> +The ELC control defines some parameters which can be changed from SW.
> +If the average CPU utilization is below a user defined threshold
> +(elc_low_threshold_percent attribute below), the user defined uncore
> +frequency floor frequency will be used (elc_floor_freq_khz attribute

Consider the following simplification:

"the user defined uncore frequency floor frequency" ->
"the user-defined uncore floor frequency"

I think it tells the same even without that first "frequency".

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

-- 
 i.

> +below) instead of hardware calculated minimum.
> +
> +Similarly in high load scenario where the CPU utilization goes above
> +the high threshold value (elc_high_threshold_percent attribute below)
> +instead of jumping to maximum uncore frequency, frequency is increased
> +in 100MHz steps. This avoids consuming unnecessarily high power
> +immediately with CPU utilization spikes.
> +
> +Attributes for efficiency latency control:
> +
> +``elc_floor_freq_khz``
> +	This attribute is used to get/set the efficiency latency floor frequency.
> +	If this variable is lower than the 'min_freq_khz', it is ignored by
> +	the firmware.
> +
> +``elc_low_threshold_percent``
> +	This attribute is used to get/set the efficiency latency control low
> +	threshold. This attribute is in percentages of CPU utilization.
> +
> +``elc_high_threshold_percent``
> +	This attribute is used to get/set the efficiency latency control high
> +	threshold. This attribute is in percentages of CPU utilization.
> +
> +``elc_high_threshold_enable``
> +	This attribute is used to enable/disable the efficiency latency control
> +	high threshold. Write '1' to enable, '0' to disable.
> +
> +Example system configuration below, which does following:
> +  * when CPU utilization is less than 10%: sets uncore frequency to 800MHz
> +  * when CPU utilization is higher than 95%: increases uncore frequency in
> +    100MHz steps, until power limit is reached
> +
> +  elc_floor_freq_khz:800000
> +  elc_high_threshold_percent:95
> +  elc_high_threshold_enable:1
> +  elc_low_threshold_percent:10
>