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Documentation: arm64: Document PMU counters access from userspace

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Add documentation 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>
Link: https://lore.kernel.org/r/20211208201124.310740-6-robh@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Raphael Gault 2021-12-08 14:11:24 -06:00 committed by Will Deacon
parent 83a7a4d643
commit aa1005d15d
1 changed files with 77 additions and 1 deletions
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78
Documentation/arm64/perf.rst
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@ -2,7 +2,10 @@
.. _perf_index:
=====================
====
Perf
====
Perf Event Attributes
=====================
@ -88,3 +91,76 @@ exclude_host. However when using !exclude_hv there is a small blackout
window at the guest entry/exit where host events are not captured.
On VHE systems there are no blackout windows.
Perf Userspace PMU Hardware Counter Access
==========================================
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 counters, the global sysctl
kernel/perf_user_access must first be enabled:
.. code-block:: sh
echo 1 > /proc/sys/kernel/perf_user_access
It is necessary to open the event using the perf tool interface with config1:1
attr bit set: 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.
Access to the PMU registers is only valid while the sequence lock is unchanged.
In particular, the PMSELR_EL0 register is zeroed each time the sequence lock is
changed.
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 counter sizes
--------------------------------------
The user can request either a 32-bit (config1:0 == 0) or 64-bit (config1:0 == 1)
counter along with userspace access. The sys_perf_event_open syscall will fail
if a 64-bit counter is requested and the hardware doesn't support 64-bit
counters. Chained events are not supported in conjunction with userspace counter
access. If a 32-bit counter is requested on hardware with 64-bit counters, then
userspace must treat the upper 32-bits read from the counter as UNKNOWN. The
'pmc_width' field in the user page will indicate the valid width of the counter
and should be used to mask the upper bits as needed.
.. Links
.. _tools/perf/arch/arm64/tests/user-events.c:
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