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Documentation: admin-guide: pm: Document uncore frequency scaling

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Added documentation to configure uncore frequency limits in Intel
Xeon processors.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Clean up the document wording ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Srinivas Pandruvada 2022-02-28 11:57:59 -08:00 committed by Rafael J. Wysocki
parent 7e57714cd0
commit a644161ba1
2 changed files with 61 additions and 0 deletions
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60
Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst Normal file
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.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>
==============================
Intel Uncore Frequency Scaling
==============================
:Copyright: |copy| 2022 Intel Corporation
:Author: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Introduction
------------
The uncore can consume significant amount of power in Intel's Xeon servers based
on the workload characteristics. To optimize the total power and improve overall
performance, SoCs have internal algorithms for scaling uncore frequency. These
algorithms monitor workload usage of uncore and set a desirable frequency.
It is possible that users have different expectations of uncore performance and
want to have control over it. The objective is similar to allowing users to set
the scaling min/max frequencies via cpufreq sysfs to improve CPU performance.
Users may have some latency sensitive workloads where they do not want any
change to uncore frequency. Also, users may have workloads which require
different core and uncore performance at distinct phases and they may want to
use both cpufreq and the uncore scaling interface to distribute power and
improve overall performance.
Sysfs Interface
---------------
To control uncore frequency, a sysfs interface is provided in the directory:
`/sys/devices/system/cpu/intel_uncore_frequency/`.
There is one directory for each package and die combination as the scope of
uncore scaling control is per die in multiple die/package SoCs or per
package for single die per package SoCs. The name represents the
scope of control. For example: 'package_00_die_00' is for package id 0 and
die 0.
Each package_*_die_* contains the following attributes:
``initial_max_freq_khz``
Out of reset, this attribute represent the maximum possible frequency.
This is a read-only attribute. If users adjust max_freq_khz,
they can always go back to maximum using the value from this attribute.
``initial_min_freq_khz``
Out of reset, this attribute represent the minimum possible frequency.
This is a read-only attribute. If users adjust min_freq_khz,
they can always go back to minimum using the value from this attribute.
``max_freq_khz``
This attribute is used to set the maximum uncore frequency.
``min_freq_khz``
This attribute is used to set the minimum uncore frequency.
``current_freq_khz``
This attribute is used to get the current uncore frequency.

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Documentation/admin-guide/pm/working-state.rst
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@ -15,3 +15,4 @@ Working-State Power Management
cpufreq_drivers
intel_epb
intel-speed-select
intel_uncore_frequency_scaling