Messages in this thread Patch in this message | | | From | Rob Herring <> | Subject | [PATCH v6 10/10] Documentation: arm64: Document PMU counters access from userspace | Date | Wed, 10 Mar 2021 17:08:37 -0700 |
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From: Raphael Gault <raphael.gault@arm.com>
Add a documentation file to describe the access to the pmu hardware counters from userspace
Signed-off-by: Raphael Gault <raphael.gault@arm.com> Signed-off-by: Rob Herring <robh@kernel.org> --- v6: - Update the chained event section with attr.config1 details v2: - Update links to test examples
Changes from Raphael's v4: - Convert to rSt - Update chained event status - Add section for heterogeneous systems --- Documentation/arm64/index.rst | 1 + .../arm64/perf_counter_user_access.rst | 60 +++++++++++++++++++ 2 files changed, 61 insertions(+) create mode 100644 Documentation/arm64/perf_counter_user_access.rst
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst index 97d65ba12a35..eb7b1cabbf08 100644 --- a/Documentation/arm64/index.rst +++ b/Documentation/arm64/index.rst @@ -18,6 +18,7 @@ ARM64 Architecture memory memory-tagging-extension perf + perf_counter_user_access pointer-authentication silicon-errata sve diff --git a/Documentation/arm64/perf_counter_user_access.rst b/Documentation/arm64/perf_counter_user_access.rst new file mode 100644 index 000000000000..a42800e72458 --- /dev/null +++ b/Documentation/arm64/perf_counter_user_access.rst @@ -0,0 +1,60 @@ +============================================= +Access to PMU hardware counter from userspace +============================================= + +Overview +-------- +The perf userspace tool relies on the PMU to monitor events. It offers an +abstraction layer over the hardware counters since the underlying +implementation is cpu-dependent. +Arm64 allows userspace tools to have access to the registers storing the +hardware counters' values directly. + +This targets specifically self-monitoring tasks in order to reduce the overhead +by directly accessing the registers without having to go through the kernel. + +How-to +------ +The focus is set on the armv8 pmuv3 which makes sure that the access to the pmu +registers is enabled and that the userspace has access to the relevant +information in order to use them. + +In order to have access to the hardware counter it is necessary to open the event +using the perf tool interface: the sys_perf_event_open syscall returns a fd which +can subsequently be used with the mmap syscall in order to retrieve a page of +memory containing information about the event. +The PMU driver uses this page to expose to the user the hardware counter's +index and other necessary data. Using this index enables the user to access the +PMU registers using the `mrs` instruction. + +The userspace access is supported in libperf using the perf_evsel__mmap() +and perf_evsel__read() functions. See `tools/lib/perf/tests/test-evsel.c`_ for +an example. + +About heterogeneous systems +--------------------------- +On heterogeneous systems such as big.LITTLE, userspace PMU counter access can +only be enabled when the tasks are pinned to a homogeneous subset of cores and +the corresponding PMU instance is opened by specifying the 'type' attribute. +The use of generic event types is not supported in this case. + +Have a look at `tools/perf/arch/arm64/tests/user-events.c`_ for an example. It +can be run using the perf tool to check that the access to the registers works +correctly from userspace: + +.. code-block:: sh + + perf test -v user + +About chained events and 64-bit counters +---------------------------------------- +Chained events are not supported in conjunction with userspace counter +access. If a 64-bit counter is requested (attr.config1:0), then +userspace access must also be requested with attr.config1:1 set. This +will disable counter chaining. The 'pmc_width' in the user page will +indicate the actual width of the counter which could be only 32-bits +depending on event and PMU features. + +.. Links +.. _tools/perf/arch/arm64/tests/user-events.c: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c -- 2.27.0
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