Documentation: admin-guide: pm: Add efficiency vs. latency tradeoff to uncore documentation

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>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240828153657.1296410-2-tero.kristo@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Author: Tero Kristo

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
+floor frequency will be used (elc_floor_freq_khz attribute 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