Add documentation for the new Perf event open parameters and the threshold_max capability file.
Reviewed-by: Anshuman Khandual anshuman.khandual@arm.com Reviewed-by: Suzuki K Poulose suzuki.poulose@arm.com Acked-by: Namhyung Kim namhyung@kernel.org Signed-off-by: James Clark james.clark@arm.com --- Documentation/arch/arm64/perf.rst | 72 +++++++++++++++++++++++++++++++ 1 file changed, 72 insertions(+)
diff --git a/Documentation/arch/arm64/perf.rst b/Documentation/arch/arm64/perf.rst index 1f87b57c2332..997fd716b82f 100644 --- a/Documentation/arch/arm64/perf.rst +++ b/Documentation/arch/arm64/perf.rst @@ -164,3 +164,75 @@ and should be used to mask the upper bits as needed. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tool... .. _tools/lib/perf/tests/test-evsel.c: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tool... + +Event Counting Threshold +========================================== + +Overview +-------- + +FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on +events whose count meets a specified threshold condition. For example if +threshold_compare is set to 2 ('Greater than or equal'), and the +threshold is set to 2, then the PMU counter will now only increment by +when an event would have previously incremented the PMU counter by 2 or +more on a single processor cycle. + +To increment by 1 after passing the threshold condition instead of the +number of events on that cycle, add the 'threshold_count' option to the +commandline. + +How-to +------ + +These are the parameters for controlling the feature: + +.. list-table:: + :header-rows: 1 + + * - Parameter + - Description + * - threshold + - Value to threshold the event by. A value of 0 means that + thresholding is disabled and the other parameters have no effect. + * - threshold_compare + - | Comparison function to use, with the following values supported: + | + | 0: Not-equal + | 1: Equals + | 2: Greater-than-or-equal + | 3: Less-than + * - threshold_count + - If this is set, count by 1 after passing the threshold condition + instead of the value of the event on this cycle. + +The threshold, threshold_compare and threshold_count values can be +provided per event, for example: + +.. code-block:: sh + + perf stat -e stall_slot/threshold=2,threshold_compare=2/ \ + -e dtlb_walk/threshold=10,threshold_compare=3,threshold_count/ + +In this example the stall_slot event will count by 2 or more on every +cycle where 2 or more stalls happen. And dtlb_walk will count by 1 on +every cycle where the number of dtlb walks were less than 10. + +The maximum supported threshold value can be read from the caps of each +PMU, for example: + +.. code-block:: sh + + cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max + + 0x000000ff + +If a value higher than this is given, then opening the event will result +in an error. The highest possible maximum is 4095, as the config field +for threshold is limited to 12 bits, and the Perf tool will refuse to +parse higher values. + +If the PMU doesn't support FEAT_PMUv3_TH, then threshold_max will read +0, and attempting to set a threshold value will also result in an error. +threshold_max will also read as 0 on aarch32 guests, even if the host +is running on hardware with the feature.