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ACPI / CPPC: Document CPPC sysfs interface
Add a file to describe the CPPC sysfs interface and steps to compute
average delivered performance using the feedback counters.
Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
[ rjw: Minor adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
authored by
Prashanth Prakash
and committed by
Rafael J. Wysocki
b382bf88
256f19d2
+69
+69
Documentation/acpi/cppc_sysfs.txt
+69
Documentation/acpi/cppc_sysfs.txt
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Collaborative Processor Performance Control (CPPC)
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CPPC defined in the ACPI spec describes a mechanism for the OS to manage the
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performance of a logical processor on a contigious and abstract performance
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scale. CPPC exposes a set of registers to describe abstract performance scale,
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to request performance levels and to measure per-cpu delivered performance.
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For more details on CPPC please refer to the ACPI specification at:
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http://uefi.org/specifications
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Some of the CPPC registers are exposed via sysfs under:
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/sys/devices/system/cpu/cpuX/acpi_cppc/
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for each cpu X
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--------------------------------------------------------------------------------
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$ ls -lR /sys/devices/system/cpu/cpu0/acpi_cppc/
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/sys/devices/system/cpu/cpu0/acpi_cppc/:
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total 0
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-r--r--r-- 1 root root 65536 Mar 5 19:38 feedback_ctrs
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-r--r--r-- 1 root root 65536 Mar 5 19:38 highest_perf
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-r--r--r-- 1 root root 65536 Mar 5 19:38 lowest_freq
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-r--r--r-- 1 root root 65536 Mar 5 19:38 lowest_nonlinear_perf
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-r--r--r-- 1 root root 65536 Mar 5 19:38 lowest_perf
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-r--r--r-- 1 root root 65536 Mar 5 19:38 nominal_freq
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-r--r--r-- 1 root root 65536 Mar 5 19:38 nominal_perf
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-r--r--r-- 1 root root 65536 Mar 5 19:38 reference_perf
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-r--r--r-- 1 root root 65536 Mar 5 19:38 wraparound_time
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--------------------------------------------------------------------------------
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* highest_perf : Highest performance of this processor (abstract scale).
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* nominal_perf : Highest sustained performance of this processor (abstract scale).
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* lowest_nonlinear_perf : Lowest performance of this processor with nonlinear
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power savings (abstract scale).
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* lowest_perf : Lowest performance of this processor (abstract scale).
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* lowest_freq : CPU frequency corresponding to lowest_perf (in MHz).
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* nominal_freq : CPU frequency corresponding to nominal_perf (in MHz).
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The above frequencies should only be used to report processor performance in
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freqency instead of abstract scale. These values should not be used for any
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functional decisions.
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* feedback_ctrs : Includes both Reference and delivered performance counter.
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Reference counter ticks up proportional to processor's reference performance.
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Delivered counter ticks up proportional to processor's delivered performance.
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* wraparound_time: Minimum time for the feedback counters to wraparound (seconds).
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* reference_perf : Performance level at which reference performance counter
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accumulates (abstract scale).
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--------------------------------------------------------------------------------
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Computing Average Delivered Performance
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Below describes the steps to compute the average performance delivered by taking
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two different snapshots of feedback counters at time T1 and T2.
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T1: Read feedback_ctrs as fbc_t1
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Wait or run some workload
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T2: Read feedback_ctrs as fbc_t2
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delivered_counter_delta = fbc_t2[del] - fbc_t1[del]
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reference_counter_delta = fbc_t2[ref] - fbc_t1[ref]
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delivered_perf = (refernce_perf x delivered_counter_delta) / reference_counter_delta