138 lines
5.3 KiB
Plaintext
138 lines
5.3 KiB
Plaintext
PMU Event Based Branches
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========================
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Event Based Branches (EBBs) are a feature which allows the hardware to
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branch directly to a specified user space address when certain events occur.
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The full specification is available in Power ISA v2.07:
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https://www.power.org/documentation/power-isa-version-2-07/
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One type of event for which EBBs can be configured is PMU exceptions. This
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document describes the API for configuring the Power PMU to generate EBBs,
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using the Linux perf_events API.
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Terminology
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-----------
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Throughout this document we will refer to an "EBB event" or "EBB events". This
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just refers to a struct perf_event which has set the "EBB" flag in its
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attr.config. All events which can be configured on the hardware PMU are
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possible "EBB events".
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Background
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----------
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When a PMU EBB occurs it is delivered to the currently running process. As such
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EBBs can only sensibly be used by programs for self-monitoring.
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It is a feature of the perf_events API that events can be created on other
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processes, subject to standard permission checks. This is also true of EBB
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events, however unless the target process enables EBBs (via mtspr(BESCR)) no
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EBBs will ever be delivered.
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This makes it possible for a process to enable EBBs for itself, but not
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actually configure any events. At a later time another process can come along
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and attach an EBB event to the process, which will then cause EBBs to be
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delivered to the first process. It's not clear if this is actually useful.
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When the PMU is configured for EBBs, all PMU interrupts are delivered to the
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user process. This means once an EBB event is scheduled on the PMU, no non-EBB
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events can be configured. This means that EBB events can not be run
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concurrently with regular 'perf' commands, or any other perf events.
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It is however safe to run 'perf' commands on a process which is using EBBs. The
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kernel will in general schedule the EBB event, and perf will be notified that
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its events could not run.
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The exclusion between EBB events and regular events is implemented using the
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existing "pinned" and "exclusive" attributes of perf_events. This means EBB
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events will be given priority over other events, unless they are also pinned.
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If an EBB event and a regular event are both pinned, then whichever is enabled
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first will be scheduled and the other will be put in error state. See the
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section below titled "Enabling an EBB event" for more information.
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Creating an EBB event
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---------------------
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To request that an event is counted using EBB, the event code should have bit
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63 set.
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EBB events must be created with a particular, and restrictive, set of
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attributes - this is so that they interoperate correctly with the rest of the
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perf_events subsystem.
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An EBB event must be created with the "pinned" and "exclusive" attributes set.
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Note that if you are creating a group of EBB events, only the leader can have
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these attributes set.
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An EBB event must NOT set any of the "inherit", "sample_period", "freq" or
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"enable_on_exec" attributes.
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An EBB event must be attached to a task. This is specified to perf_event_open()
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by passing a pid value, typically 0 indicating the current task.
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All events in a group must agree on whether they want EBB. That is all events
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must request EBB, or none may request EBB.
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EBB events must specify the PMC they are to be counted on. This ensures
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userspace is able to reliably determine which PMC the event is scheduled on.
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Enabling an EBB event
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---------------------
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Once an EBB event has been successfully opened, it must be enabled with the
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perf_events API. This can be achieved either via the ioctl() interface, or the
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prctl() interface.
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However, due to the design of the perf_events API, enabling an event does not
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guarantee that it has been scheduled on the PMU. To ensure that the EBB event
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has been scheduled on the PMU, you must perform a read() on the event. If the
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read() returns EOF, then the event has not been scheduled and EBBs are not
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enabled.
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This behaviour occurs because the EBB event is pinned and exclusive. When the
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EBB event is enabled it will force all other non-pinned events off the PMU. In
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this case the enable will be successful. However if there is already an event
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pinned on the PMU then the enable will not be successful.
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Reading an EBB event
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--------------------
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It is possible to read() from an EBB event. However the results are
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meaningless. Because interrupts are being delivered to the user process the
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kernel is not able to count the event, and so will return a junk value.
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Closing an EBB event
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--------------------
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When an EBB event is finished with, you can close it using close() as for any
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regular event. If this is the last EBB event the PMU will be deconfigured and
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no further PMU EBBs will be delivered.
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EBB Handler
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-----------
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The EBB handler is just regular userspace code, however it must be written in
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the style of an interrupt handler. When the handler is entered all registers
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are live (possibly) and so must be saved somehow before the handler can invoke
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other code.
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It's up to the program how to handle this. For C programs a relatively simple
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option is to create an interrupt frame on the stack and save registers there.
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Fork
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----
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EBB events are not inherited across fork. If the child process wishes to use
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EBBs it should open a new event for itself. Similarly the EBB state in
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BESCR/EBBHR/EBBRR is cleared across fork().
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