On 11/24/23 15:58, James Clark wrote: > Add documentation for the new Perf event open parameters and > the threshold_max capability file. > > Signed-off-by: James Clark <james.clark@xxxxxxx> > --- > 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..41eee68951ff 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/tools/perf/arch/arm64/tests/user-events.c > .. _tools/lib/perf/tests/test-evsel.c: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c > + > +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 it will be silently clamped > +to the maximum. 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 both threshold and threshold_compare will be silently ignored. > +threshold_max will also read as 0 on aarch32 guests, even if the host > +is running on hardware with the feature. Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
