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diff --git a/Documentation/arch/arm64/perf.rst b/Documentation/arch/arm64/perf.rst new file mode 100644 index 000000000000..1f87b57c2332 --- /dev/null +++ b/Documentation/arch/arm64/perf.rst @@ -0,0 +1,166 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _perf_index: + +==== +Perf +==== + +Perf Event Attributes +===================== + +:Author: Andrew Murray <andrew.murray@arm.com> +:Date: 2019-03-06 + +exclude_user +------------ + +This attribute excludes userspace. + +Userspace always runs at EL0 and thus this attribute will exclude EL0. + + +exclude_kernel +-------------- + +This attribute excludes the kernel. + +The kernel runs at EL2 with VHE and EL1 without. Guest kernels always run +at EL1. + +For the host this attribute will exclude EL1 and additionally EL2 on a VHE +system. + +For the guest this attribute will exclude EL1. Please note that EL2 is +never counted within a guest. + + +exclude_hv +---------- + +This attribute excludes the hypervisor. + +For a VHE host this attribute is ignored as we consider the host kernel to +be the hypervisor. + +For a non-VHE host this attribute will exclude EL2 as we consider the +hypervisor to be any code that runs at EL2 which is predominantly used for +guest/host transitions. + +For the guest this attribute has no effect. Please note that EL2 is +never counted within a guest. + + +exclude_host / exclude_guest +---------------------------- + +These attributes exclude the KVM host and guest, respectively. + +The KVM host may run at EL0 (userspace), EL1 (non-VHE kernel) and EL2 (VHE +kernel or non-VHE hypervisor). + +The KVM guest may run at EL0 (userspace) and EL1 (kernel). + +Due to the overlapping exception levels between host and guests we cannot +exclusively rely on the PMU's hardware exception filtering - therefore we +must enable/disable counting on the entry and exit to the guest. This is +performed differently on VHE and non-VHE systems. + +For non-VHE systems we exclude EL2 for exclude_host - upon entering and +exiting the guest we disable/enable the event as appropriate based on the +exclude_host and exclude_guest attributes. + +For VHE systems we exclude EL1 for exclude_guest and exclude both EL0,EL2 +for exclude_host. Upon entering and exiting the guest we modify the event +to include/exclude EL0 as appropriate based on the exclude_host and +exclude_guest attributes. + +The statements above also apply when these attributes are used within a +non-VHE guest however please note that EL2 is never counted within a guest. + + +Accuracy +-------- + +On non-VHE hosts we enable/disable counters on the entry/exit of host/guest +transition at EL2 - however there is a period of time between +enabling/disabling the counters and entering/exiting the guest. We are +able to eliminate counters counting host events on the boundaries of guest +entry/exit when counting guest events by filtering out EL2 for +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 |