lib/libpmc/pmc.3

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.Dd August 10, 2021
.Dt PMC 3
.Os
.Sh NAME
.Nm pmc
.Nd library for accessing hardware performance monitoring counters
.Sh LIBRARY
.Lb libpmc
.Sh SYNOPSIS
.In pmc.h
.Sh DESCRIPTION
The
.Lb libpmc
provides a programming interface that allows applications to use
hardware performance counters to gather performance data about
specific processes or for the system as a whole.
The library is implemented using the lower-level facilities offered by
the
.Xr hwpmc 4
driver.
.Ss Key Concepts
Performance monitoring counters (PMCs) are represented by the library
using a software abstraction.
These
.Dq abstract
PMCs can have two scopes:
.Bl -bullet
.It
System scope.
These PMCs measure events in a whole-system manner, i.e., independent
of the currently executing thread.
System scope PMCs are allocated on specific CPUs and do not
migrate between CPUs.
Non-privileged process are allowed to allocate system scope PMCs if the
.Xr hwpmc 4
sysctl tunable:
.Va security.bsd.unprivileged_syspmcs
is non-zero.
.It
Process scope.
These PMCs only measure hardware events when the processes they are
attached to are executing on a CPU.
In an SMP system, process scope PMCs migrate between CPUs along with
their target processes.
.El
.Pp
Orthogonal to PMC scope, PMCs may be allocated in one of two
operational modes:
.Bl -bullet
.It
Counting PMCs measure events according to their scope
(system or process).
The application needs to explicitly read these counters
to retrieve their value.
.It
Sampling PMCs cause the CPU to be periodically interrupted
and information about its state of execution to be collected.
Sampling PMCs are used to profile specific processes and kernel
threads or to profile the system as a whole.
.El
.Pp
The scope and operational mode for a software PMC are specified at
PMC allocation time.
An application is allowed to allocate multiple PMCs subject
to availability of hardware resources.
.Pp
The library uses human-readable strings to name the event being
measured by hardware.
The syntax used for specifying a hardware event along with additional
event specific qualifiers (if any) is described in detail in section
.Sx "EVENT SPECIFIERS"
below.
.Pp
PMCs are associated with the process that allocated them and
will be automatically reclaimed by the system when the process exits.
Additionally, process-scope PMCs have to be attached to one or more
target processes before they can perform measurements.
A process-scope PMC may be attached to those target processes
that its owner process would otherwise be permitted to debug.
An owner process may attach PMCs to itself allowing
it to measure its own behavior.
Additionally, on some machine architectures, such self-attached PMCs
may be read cheaply using specialized instructions supported by the
processor.
.Pp
Certain kinds of PMCs require that a log file be configured before
they may be started.
These include:
.Bl -bullet
.It
System scope sampling PMCs.
.It
Process scope sampling PMCs.
.It
Process scope counting PMCs that have been configured to report PMC
readings on process context switches or process exits.
.El
.Pp
Up to one log file may be configured per owner process.
Events logged to a log file may be subsequently analyzed using the
.Xr pmclog 3
family of functions.
.Ss Supported CPUs
The CPUs known to the PMC library are named by the
.Vt "enum pmc_cputype"
enumeration.
Supported CPUs include:
.Pp
.Bl -tag -width "Li PMC_CPU_INTEL_CORE2" -compact
.It Li PMC_CPU_AMD_K7
.Tn "AMD Athlon"
CPUs.
.It Li PMC_CPU_AMD_K8
.Tn "AMD Athlon64"
CPUs.
.It Li PMC_CPU_INTEL_ATOM
.Tn Intel
.Tn Atom
CPUs and other CPUs conforming to version 3 of the
.Tn Intel
performance measurement architecture.
.It Li PMC_CPU_INTEL_CORE
.Tn Intel
.Tn Core Solo
and
.Tn Core Duo
CPUs, and other CPUs conforming to version 1 of the
.Tn Intel
performance measurement architecture.
.It Li PMC_CPU_INTEL_CORE2
.Tn Intel
.Tn "Core2 Solo" ,
.Tn "Core2 Duo"
and
.Tn "Core2 Extreme"
CPUs, and other CPUs conforming to version 2 of the
.Tn Intel
performance measurement architecture.
.El
.Ss Supported PMCs
PMC supported by this library are named by the
.Vt enum pmc_class
enumeration.
Supported PMC kinds include:
.Pp
.Bl -tag -width "Li PMC_CLASS_IAF" -compact
.It Li PMC_CLASS_IAF
Fixed function hardware counters presents in CPUs conforming to the
.Tn Intel
performance measurement architecture version 2 and later.
.It Li PMC_CLASS_IAP
Programmable hardware counters present in CPUs conforming to the
.Tn Intel
performance measurement architecture version 1 and later.
.It Li PMC_CLASS_K7
Programmable hardware counters present in
.Tn "AMD Athlon"
CPUs.
.It Li PMC_CLASS_K8
Programmable hardware counters present in
.Tn "AMD Athlon64"
CPUs.
.It Li PMC_CLASS_TSC
The timestamp counter on i386 and amd64 architecture CPUs.
.It Li PMC_CLASS_SOFT
Software events.
.El
.Ss PMC Capabilities
Capabilities of performance monitoring hardware are denoted using
the
.Vt "enum pmc_caps"
enumeration.
Supported capabilities include:
.Pp
.Bl -tag -width "Li PMC_CAP_INTERRUPT" -compact
.It Li PMC_CAP_CASCADE
The ability to cascade counters.
.It Li PMC_CAP_EDGE
The ability to count negated to asserted transitions of the hardware
conditions being probed for.
.It Li PMC_CAP_INTERRUPT
The ability to interrupt the CPU.
.It Li PMC_CAP_INVERT
The ability to invert the sense of the hardware conditions being
measured.
.It Li PMC_CAP_PRECISE
The ability to perform precise sampling.
.It Li PMC_CAP_QUALIFIER
The hardware allows monitored to be further qualified in some
system dependent way.
.It Li PMC_CAP_READ
The ability to read from performance counters.
.It Li PMC_CAP_SYSTEM
The ability to restrict counting of hardware events to when the CPU is
running privileged code.
.It Li PMC_CAP_THRESHOLD
The ability to ignore simultaneous hardware events below a
programmable threshold.
.It Li PMC_CAP_USER
The ability to restrict counting of hardware events to those when the
CPU is running unprivileged code.
.It Li PMC_CAP_WRITE
The ability to write to performance counters.
.El
.Ss CPU Naming Conventions
CPUs are named using small integers from zero up to, but
excluding, the value returned by function
.Fn pmc_ncpu .
On platforms supporting sparsely numbered CPUs not all the numbers in
this range will denote valid CPUs.
Operations on non-existent CPUs will return an error.
.Ss Functional Grouping of the API
This section contains a brief overview of the available functionality
in the PMC library.
Each function listed here is described further in its own manual page.
.Bl -tag -width 2n
.It Administration
.Bl -tag -width 6n -compact
.It Fn pmc_disable , Fn pmc_enable
Administratively disable (enable) specific performance monitoring
counter hardware.
Counters that are disabled will not be available to applications to
use.
.El
.It "Convenience Functions"
.Bl -tag -width 6n -compact
.It Fn pmc_event_names_of_class
Returns a list of event names supported by a given PMC type.
.It Fn pmc_name_of_capability
Convert a
.Dv PMC_CAP_*
flag to a human-readable string.
.It Fn pmc_name_of_class
Convert a
.Dv PMC_CLASS_*
constant to a human-readable string.
.It Fn pmc_name_of_cputype
Return a human-readable name for a CPU type.
.It Fn pmc_name_of_disposition
Return a human-readable string describing a PMC's disposition.
.It Fn pmc_name_of_event
Convert a numeric event code to a human-readable string.
.It Fn pmc_name_of_mode
Convert a
.Dv PMC_MODE_*
constant to a human-readable name.
.It Fn pmc_name_of_state
Return a human-readable string describing a PMC's current state.
.El
.It "Library Initialization"
.Bl -tag -width 6n -compact
.It Fn pmc_init
Initialize the library.
This function must be called before any other library function.
.El
.It "Log File Handling"
.Bl -tag -width 6n -compact
.It Fn pmc_configure_logfile
Configure a log file for
.Xr hwpmc 4
to write logged events to.
.It Fn pmc_flush_logfile
Flush all pending log data in
.Xr hwpmc 4 Ns Ap s
buffers.
.It Fn pmc_close_logfile
Flush all pending log data and close
.Xr hwpmc 4 Ns Ap s
side of the stream.
.It Fn pmc_writelog
Append arbitrary user data to the current log file.
.El
.It "PMC Management"
.Bl -tag -width 6n -compact
.It Fn pmc_allocate , Fn pmc_release
Allocate (free) a PMC.
.It Fn pmc_attach , Fn pmc_detach
Attach (detach) a process scope PMC to a target.
.It Fn pmc_read , Fn pmc_write , Fn pmc_rw
Read (write) a value from (to) a PMC.
.It Fn pmc_start , Fn pmc_stop
Start (stop) a software PMC.
.It Fn pmc_set
Set the reload value for a sampling PMC.
.El
.It "Queries"
.Bl -tag -width 6n -compact
.It Fn pmc_capabilities
Retrieve the capabilities for a given PMC.
.It Fn pmc_cpuinfo
Retrieve information about the CPUs and PMC hardware present in the
system.
.It Fn pmc_get_driver_stats
Retrieve statistics maintained by
.Xr hwpmc 4 .
.It Fn pmc_ncpu
Determine the greatest possible CPU number on the system.
.It Fn pmc_npmc
Return the number of hardware PMCs present in a given CPU.
.It Fn pmc_pmcinfo
Return information about the state of a given CPU's PMCs.
.It Fn pmc_width
Determine the width of a hardware counter in bits.
.El
.It "x86 Architecture Specific API"
.Bl -tag -width 6n -compact
.It Fn pmc_get_msr
Returns the processor model specific register number
associated with
.Fa pmc .
Applications may then use the x86
.Ic RDPMC
instruction to directly read the contents of the PMC.
.El
.El
.Ss Signal Handling Requirements
Applications using PMCs are required to handle the following signals:
.Bl -tag -width ".Dv SIGBUS"
.It Dv SIGBUS
When the
.Xr hwpmc 4
module is unloaded using
.Xr kldunload 8 ,
processes that have PMCs allocated to them will be sent a
.Dv SIGBUS
signal.
.It Dv SIGIO
The
.Xr hwpmc 4
driver will send a PMC owning process a
.Dv SIGIO
signal if:
.Bl -bullet
.It
If any process-mode PMC allocated by it loses all its
target processes.
.It
If the driver encounters an error when writing log data to a
configured log file.
This error may be retrieved by a subsequent call to
.Fn pmc_flush_logfile .
.El
.El
.Ss Typical Program Flow
.Bl -enum
.It
An application would first invoke function
.Fn pmc_init
to allow the library to initialize itself.
.It
Signal handling would then be set up.
.It
Next the application would allocate the PMCs it desires using function
.Fn pmc_allocate .
.It
Initial values for PMCs may be set using function
.Fn pmc_set .
.It
If a log file is necessary for the PMCs to work, it would
be configured using function
.Fn pmc_configure_logfile .
.It
Process scope PMCs would then be attached to their target processes
using function
.Fn pmc_attach .
.It
The PMCs would then be started using function
.Fn pmc_start .
.It
Once started, the values of counting PMCs may be read using function
.Fn pmc_read .
For PMCs that write events to the log file, this logged data would be
read and parsed using the
.Xr pmclog 3
family of functions.
.It
PMCs are stopped using function
.Fn pmc_stop ,
and process scope PMCs are detached from their targets using
function
.Fn pmc_detach .
.It
Before the process exits, its may release its PMCs using function
.Fn pmc_release .
Any configured log file may be closed using function
.Fn pmc_configure_logfile .
.El
.Sh EVENT SPECIFIERS
Event specifiers are strings comprising of an event name, followed by
optional parameters modifying the semantics of the hardware event
being probed.
Event names are PMC architecture dependent, but the PMC library defines
machine independent aliases for commonly used events.
.Pp
Event specifiers spellings are case-insensitive and space characters,
periods, underscores and hyphens are considered equivalent to each other.
Thus the event specifiers
.Qq "Example Event" ,
.Qq "example-event" ,
and
.Qq "EXAMPLE_EVENT"
are equivalent.
.Ss PMC Architecture Dependent Events
PMC architecture dependent event specifiers are described in the
following manual pages:
.Bl -column " PMC_CLASS_TSC " "MANUAL PAGE "
.It Em "PMC Class"      Ta Em "Manual Page"
.It Li PMC_CLASS_IAF    Ta Xr pmc.iaf 3
.It Li PMC_CLASS_IAP    Ta Xr pmc.atom 3 , Xr pmc.core 3 , Xr pmc.core2 3
.It Li PMC_CLASS_K7     Ta Xr pmc.k7 3
.It Li PMC_CLASS_K8     Ta Xr pmc.k8 3
.It Li PMC_CLASS_TSC    Ta Xr pmc.tsc 3
.El
.Ss Event Name Aliases
Event name aliases are PMC-independent names for commonly used events.
The following aliases are known to this version of the
.Nm pmc
library:
.Bl -tag -width indent
.It Li branches
Measure the number of branches retired.
.It Li branch-mispredicts
Measure the number of retired branches that were mispredicted.
.It Li cycles
Measure processor cycles.
This event is implemented using the processor's Time Stamp Counter
register.
.It Li dc-misses
Measure the number of data cache misses.
.It Li ic-misses
Measure the number of instruction cache misses.
.It Li instructions
Measure the number of instructions retired.
.It Li interrupts
Measure the number of interrupts seen.
.It Li unhalted-cycles
Measure the number of cycles the processor is not in a halted
or sleep state.
.El
.Sh COMPATIBILITY
The interface between the
.Nm pmc
library and the
.Xr hwpmc 4
driver is intended to be private to the implementation and may
change.
In order to ease forward compatibility with future versions of the
.Xr hwpmc 4
driver, applications are urged to dynamically link with the
.Nm pmc
library.
.Pp
The
.Nm pmc
API is
.Ud
.Sh SEE ALSO
.Xr pmc.atom 3 ,
.Xr pmc.core 3 ,
.Xr pmc.core2 3 ,
.Xr pmc.haswell 3 ,
.Xr pmc.haswelluc 3 ,
.Xr pmc.haswellxeon 3 ,
.Xr pmc.iaf 3 ,
.Xr pmc.ivybridge 3 ,
.Xr pmc.ivybridgexeon 3 ,
.Xr pmc.k7 3 ,
.Xr pmc.k8 3 ,
.Xr pmc.sandybridge 3 ,
.Xr pmc.sandybridgeuc 3 ,
.Xr pmc.sandybridgexeon 3 ,
.Xr pmc.soft 3 ,
.Xr pmc.tsc 3 ,
.Xr pmc.westmere 3 ,
.Xr pmc.westmereuc 3 ,
.Xr pmc_allocate 3 ,
.Xr pmc_attach 3 ,
.Xr pmc_capabilities 3 ,
.Xr pmc_configure_logfile 3 ,
.Xr pmc_disable 3 ,
.Xr pmc_event_names_of_class 3 ,
.Xr pmc_get_driver_stats 3 ,
.Xr pmc_get_msr 3 ,
.Xr pmc_init 3 ,
.Xr pmc_name_of_capability 3 ,
.Xr pmc_read 3 ,
.Xr pmc_set 3 ,
.Xr pmc_start 3 ,
.Xr pmclog 3 ,
.Xr hwpmc 4 ,
.Xr pmccontrol 8 ,
.Xr pmcstat 8
.Sh HISTORY
The
.Nm pmc
library first appeared in
.Fx 6.0 .
.Sh AUTHORS
The
.Lb libpmc
library was written by
.An Joseph Koshy Aq Mt jkoshy@FreeBSD.org .