Documentation/arm64/perf.rst - linux - Git at Google

 .. 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
	.. 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