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author | Will Deacon <will@kernel.org> | 2021-12-14 13:42:22 +0000 |
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committer | Will Deacon <will@kernel.org> | 2021-12-14 13:42:22 +0000 |
commit | 8bd09b41b82f60a9a4439c2a641ffb542a10c237 (patch) | |
tree | 786537ba75be02aff449e7c0d3e4d6324bbdfdfe /Documentation | |
parent | 1879a61f4ad80c7b8cb27d52154ac253e74ebe82 (diff) | |
parent | aa1005d15d2aee10e5b93a25db076c47e05c4efa (diff) | |
download | linux-8bd09b41b82f60a9a4439c2a641ffb542a10c237.tar.bz2 |
Merge branch 'for-next/perf-user-counter-access' into for-next/perf
* for-next/perf-user-counter-access:
Documentation: arm64: Document PMU counters access from userspace
arm64: perf: Enable PMU counter userspace access for perf event
arm64: perf: Add userspace counter access disable switch
perf: Add a counter for number of user access events in context
x86: perf: Move RDPMC event flag to a common definition
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/admin-guide/sysctl/kernel.rst | 11 | ||||
-rw-r--r-- | Documentation/arm64/perf.rst | 78 |
2 files changed, 88 insertions, 1 deletions
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index 0e486f41185e..d359bcfadd39 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -905,6 +905,17 @@ enabled, otherwise writing to this file will return ``-EBUSY``. The default value is 8. +perf_user_access (arm64 only) +================================= + +Controls user space access for reading perf event counters. When set to 1, +user space can read performance monitor counter registers directly. + +The default value is 0 (access disabled). + +See Documentation/arm64/perf.rst for more information. + + pid_max ======= diff --git a/Documentation/arm64/perf.rst b/Documentation/arm64/perf.rst index b567f177d385..1f87b57c2332 100644 --- a/Documentation/arm64/perf.rst +++ b/Documentation/arm64/perf.rst @@ -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 |