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

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Message-ID: <dabdc81e-d743-6402-f87a-dee2d6b906b8@linux.intel.com>
Date: Fri, 23 Aug 2024 15:28:10 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Tero Kristo <tero.kristo@...ux.intel.com>
cc: srinivas.pandruvada@...ux.intel.com, Hans de Goede <hdegoede@...hat.com>, 
    platform-driver-x86@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Documentation: admin-guide: pm: Add efficiency vs.
 latency tradeoff to uncore documentation

On Wed, 21 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@...ux.intel.com>
> ---
>  .../pm/intel_uncore_frequency_scaling.rst     | 51 +++++++++++++++++++
>  1 file changed, 51 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..fb83aa2b744e 100644
> --- a/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
> +++ b/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
> @@ -113,3 +113,54 @@ 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

Does this section even cover the "tradeoff" part in its body? Why not call 
it directly "Control" after ELC?

> +-------------------------------
> +
> +In the realm of high-performance computing, particularly with Xeon
> +processors, managing uncore frequency is an important aspect of system
> +optimization. Traditionally, the uncore frequency is ramped up rapidly
> +in high load scenarios. While this strategy achieves low latency, which
> +is crucial for time-sensitive computations, it does not necessarily yield
> +the best performance per watt, —a key metric for energy efficiency and
> +operational cost savings.

This entire paragraph feels more prose or history book than documentation 
text. I'd suggest using something that goes more directly into the point
about what ELC brings to the table (I suppose the goal is "performance 
per watt" optimization, even that goal is only implied by the current 
text, not explicitly stated as the goal here).

-- 
 i.

> +The Efficiency vs. Latency Control (ELC) feature allows user to influence
> +the uncore frequency scaling algorithm. Hardware monitors the average CPU
> +utilization across all cores at regular intervals. If the average CPU
> +utilization is below a user defined threshold (elc_low_threshold_percent),
> +the user defined uncore frequency floor frequency will be used
> +(elc_floor_freq_khz), minimizing latency. Similarly in high load scenario
> +where the CPU utilization goes above the high threshold value
> +(elc_high_threshold_percent) instead of jumping to maximum uncore
> +frequency, uncore frequency is increased in 100MHz steps until the power
> +limit is reached.
> +
> +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
>