From: Rob Herring <robh@kernel.org>
To: Will Deacon <will@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Ian Rogers <irogers@google.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
linux-kernel@vger.kernel.org, honnappa.nagarahalli@arm.com,
Raphael Gault <raphael.gault@arm.com>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>,
Namhyung Kim <namhyung@kernel.org>,
linux-arm-kernel@lists.infradead.org
Subject: [PATCH v2 8/9] Documentation: arm64: Document PMU counters access from userspace
Date: Fri, 28 Aug 2020 14:56:13 -0600 [thread overview]
Message-ID: <20200828205614.3391252-9-robh@kernel.org> (raw)
In-Reply-To: <20200828205614.3391252-1-robh@kernel.org>
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>
---
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 | 56 +++++++++++++++++++
2 files changed, 57 insertions(+)
create mode 100644 Documentation/arm64/perf_counter_user_access.rst
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index d9665d83c53a..c712a08e7627 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -15,6 +15,7 @@ ARM64 Architecture
legacy_instructions
memory
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..e49e141f10cc
--- /dev/null
+++ b/Documentation/arm64/perf_counter_user_access.rst
@@ -0,0 +1,56 @@
+=============================================
+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
+--------------------
+Chained events are not supported in userspace. If a 64-bit counter is requested,
+userspace access will only be enabled if the underlying counter is 64-bit.
+
+.. 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
--
2.25.1
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2020-08-28 20:59 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-28 20:56 [PATCH v2 0/9] libperf and arm64 userspace counter access support Rob Herring
2020-08-28 20:56 ` [PATCH v2 1/9] arm64: pmu: Add hook to handle pmu-related undefined instructions Rob Herring
2020-08-28 20:56 ` [PATCH v2 2/9] arm64: pmu: Add function implementation to update event index in userpage Rob Herring
2020-08-28 20:56 ` [PATCH v2 3/9] arm64: perf: Enable pmu counter direct access for perf event on armv8 Rob Herring
2020-08-28 20:56 ` [PATCH v2 4/9] tools/include: Add an initial math64.h Rob Herring
2020-08-28 20:56 ` [PATCH v2 5/9] libperf: Add support for user space counter access Rob Herring
2020-08-31 9:11 ` Jiri Olsa
2020-09-02 16:58 ` Rob Herring
2020-08-31 9:11 ` Jiri Olsa
2020-09-02 17:01 ` Rob Herring
2020-09-02 18:07 ` Ian Rogers
2020-09-02 19:48 ` Rob Herring
2020-09-04 5:51 ` Ian Rogers
2020-08-28 20:56 ` [PATCH v2 6/9] libperf: Add arm64 support to perf_mmap__read_self() Rob Herring
2020-08-28 20:56 ` [PATCH v2 7/9] perf: arm64: Add test for userspace counter access on heterogeneous systems Rob Herring
2020-08-28 20:56 ` Rob Herring [this message]
2020-08-28 20:56 ` [PATCH v2 9/9] perf: Remove x86 specific rdpmc test Rob Herring
2020-08-31 9:11 ` Jiri Olsa
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200828205614.3391252-9-robh@kernel.org \
--to=robh@kernel.org \
--cc=Jonathan.Cameron@huawei.com \
--cc=acme@kernel.org \
--cc=alexander.shishkin@linux.intel.com \
--cc=catalin.marinas@arm.com \
--cc=honnappa.nagarahalli@arm.com \
--cc=irogers@google.com \
--cc=jolsa@redhat.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mingo@redhat.com \
--cc=namhyung@kernel.org \
--cc=peterz@infradead.org \
--cc=raphael.gault@arm.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).