perf.rst (6095B)
1.. SPDX-License-Identifier: GPL-2.0 2 3.. _perf_index: 4 5==== 6Perf 7==== 8 9Perf Event Attributes 10===================== 11 12:Author: Andrew Murray <andrew.murray@arm.com> 13:Date: 2019-03-06 14 15exclude_user 16------------ 17 18This attribute excludes userspace. 19 20Userspace always runs at EL0 and thus this attribute will exclude EL0. 21 22 23exclude_kernel 24-------------- 25 26This attribute excludes the kernel. 27 28The kernel runs at EL2 with VHE and EL1 without. Guest kernels always run 29at EL1. 30 31For the host this attribute will exclude EL1 and additionally EL2 on a VHE 32system. 33 34For the guest this attribute will exclude EL1. Please note that EL2 is 35never counted within a guest. 36 37 38exclude_hv 39---------- 40 41This attribute excludes the hypervisor. 42 43For a VHE host this attribute is ignored as we consider the host kernel to 44be the hypervisor. 45 46For a non-VHE host this attribute will exclude EL2 as we consider the 47hypervisor to be any code that runs at EL2 which is predominantly used for 48guest/host transitions. 49 50For the guest this attribute has no effect. Please note that EL2 is 51never counted within a guest. 52 53 54exclude_host / exclude_guest 55---------------------------- 56 57These attributes exclude the KVM host and guest, respectively. 58 59The KVM host may run at EL0 (userspace), EL1 (non-VHE kernel) and EL2 (VHE 60kernel or non-VHE hypervisor). 61 62The KVM guest may run at EL0 (userspace) and EL1 (kernel). 63 64Due to the overlapping exception levels between host and guests we cannot 65exclusively rely on the PMU's hardware exception filtering - therefore we 66must enable/disable counting on the entry and exit to the guest. This is 67performed differently on VHE and non-VHE systems. 68 69For non-VHE systems we exclude EL2 for exclude_host - upon entering and 70exiting the guest we disable/enable the event as appropriate based on the 71exclude_host and exclude_guest attributes. 72 73For VHE systems we exclude EL1 for exclude_guest and exclude both EL0,EL2 74for exclude_host. Upon entering and exiting the guest we modify the event 75to include/exclude EL0 as appropriate based on the exclude_host and 76exclude_guest attributes. 77 78The statements above also apply when these attributes are used within a 79non-VHE guest however please note that EL2 is never counted within a guest. 80 81 82Accuracy 83-------- 84 85On non-VHE hosts we enable/disable counters on the entry/exit of host/guest 86transition at EL2 - however there is a period of time between 87enabling/disabling the counters and entering/exiting the guest. We are 88able to eliminate counters counting host events on the boundaries of guest 89entry/exit when counting guest events by filtering out EL2 for 90exclude_host. However when using !exclude_hv there is a small blackout 91window at the guest entry/exit where host events are not captured. 92 93On VHE systems there are no blackout windows. 94 95Perf Userspace PMU Hardware Counter Access 96========================================== 97 98Overview 99-------- 100The perf userspace tool relies on the PMU to monitor events. It offers an 101abstraction layer over the hardware counters since the underlying 102implementation is cpu-dependent. 103Arm64 allows userspace tools to have access to the registers storing the 104hardware counters' values directly. 105 106This targets specifically self-monitoring tasks in order to reduce the overhead 107by directly accessing the registers without having to go through the kernel. 108 109How-to 110------ 111The focus is set on the armv8 PMUv3 which makes sure that the access to the pmu 112registers is enabled and that the userspace has access to the relevant 113information in order to use them. 114 115In order to have access to the hardware counters, the global sysctl 116kernel/perf_user_access must first be enabled: 117 118.. code-block:: sh 119 120 echo 1 > /proc/sys/kernel/perf_user_access 121 122It is necessary to open the event using the perf tool interface with config1:1 123attr bit set: the sys_perf_event_open syscall returns a fd which can 124subsequently be used with the mmap syscall in order to retrieve a page of memory 125containing information about the event. The PMU driver uses this page to expose 126to the user the hardware counter's index and other necessary data. Using this 127index enables the user to access the PMU registers using the `mrs` instruction. 128Access to the PMU registers is only valid while the sequence lock is unchanged. 129In particular, the PMSELR_EL0 register is zeroed each time the sequence lock is 130changed. 131 132The userspace access is supported in libperf using the perf_evsel__mmap() 133and perf_evsel__read() functions. See `tools/lib/perf/tests/test-evsel.c`_ for 134an example. 135 136About heterogeneous systems 137--------------------------- 138On heterogeneous systems such as big.LITTLE, userspace PMU counter access can 139only be enabled when the tasks are pinned to a homogeneous subset of cores and 140the corresponding PMU instance is opened by specifying the 'type' attribute. 141The use of generic event types is not supported in this case. 142 143Have a look at `tools/perf/arch/arm64/tests/user-events.c`_ for an example. It 144can be run using the perf tool to check that the access to the registers works 145correctly from userspace: 146 147.. code-block:: sh 148 149 perf test -v user 150 151About chained events and counter sizes 152-------------------------------------- 153The user can request either a 32-bit (config1:0 == 0) or 64-bit (config1:0 == 1) 154counter along with userspace access. The sys_perf_event_open syscall will fail 155if a 64-bit counter is requested and the hardware doesn't support 64-bit 156counters. Chained events are not supported in conjunction with userspace counter 157access. If a 32-bit counter is requested on hardware with 64-bit counters, then 158userspace must treat the upper 32-bits read from the counter as UNKNOWN. The 159'pmc_width' field in the user page will indicate the valid width of the counter 160and should be used to mask the upper bits as needed. 161 162.. Links 163.. _tools/perf/arch/arm64/tests/user-events.c: 164 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c 165.. _tools/lib/perf/tests/test-evsel.c: 166 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c