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Documentation: admin-guide: pm: Document uncore frequency scaling
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>
authored by
Srinivas Pandruvada
and committed by
Rafael J. Wysocki
a644161b
7e57714c
+61
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Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
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Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
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.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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==============================
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Intel Uncore Frequency Scaling
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==============================
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:Copyright: |copy| 2022 Intel Corporation
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:Author: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
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Introduction
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------------
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The uncore can consume significant amount of power in Intel's Xeon servers based
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on the workload characteristics. To optimize the total power and improve overall
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performance, SoCs have internal algorithms for scaling uncore frequency. These
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algorithms monitor workload usage of uncore and set a desirable frequency.
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It is possible that users have different expectations of uncore performance and
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want to have control over it. The objective is similar to allowing users to set
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the scaling min/max frequencies via cpufreq sysfs to improve CPU performance.
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Users may have some latency sensitive workloads where they do not want any
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change to uncore frequency. Also, users may have workloads which require
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different core and uncore performance at distinct phases and they may want to
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use both cpufreq and the uncore scaling interface to distribute power and
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improve overall performance.
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Sysfs Interface
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---------------
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To control uncore frequency, a sysfs interface is provided in the directory:
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`/sys/devices/system/cpu/intel_uncore_frequency/`.
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There is one directory for each package and die combination as the scope of
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uncore scaling control is per die in multiple die/package SoCs or per
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package for single die per package SoCs. The name represents the
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scope of control. For example: 'package_00_die_00' is for package id 0 and
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die 0.
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Each package_*_die_* contains the following attributes:
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``initial_max_freq_khz``
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Out of reset, this attribute represent the maximum possible frequency.
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This is a read-only attribute. If users adjust max_freq_khz,
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they can always go back to maximum using the value from this attribute.
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``initial_min_freq_khz``
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Out of reset, this attribute represent the minimum possible frequency.
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This is a read-only attribute. If users adjust min_freq_khz,
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they can always go back to minimum using the value from this attribute.
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``max_freq_khz``
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This attribute is used to set the maximum uncore frequency.
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``min_freq_khz``
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This attribute is used to set the minimum uncore frequency.
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``current_freq_khz``
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This attribute is used to get the current uncore frequency.