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HWPMC(4) FreeBSD Kernel Interfaces Manual HWPMC(4)
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[править] NAME
hwpmc -- Hardware Performance Monitoring Counter support
[править] SYNOPSIS
options HWPMC_HOOKS
device hwpmc
Additionally, for i386 systems:
device apic
[править] DESCRIPTION
The hwpmc driver virtualizes the hardware performance monitoring facili-
ties in modern CPUs and provides support for using these facilities from
user level processes.
The driver supports multi-processor systems.
PMCs are allocated using the PMC_OP_PMCALLOCATE request. A successful
PMC_OP_PMCALLOCATE request will return a handle to the requesting
process. Subsequent operations on the allocated PMC use this handle to
denote the specific PMC. A process that has successfully allocated a PMC
is termed an ``owner process.
PMCs may be allocated with process or system scope.
Process-scope The PMC is active only when a thread belonging to a
process it is attached to is scheduled on a CPU.
System-scope The PMC operates independently of processes and measures
hardware events for the system as a whole.
PMCs may be allocated for counting or for sampling:
Counting In counting modes, the PMCs count hardware events. These
counts are retrievable using the PMC_OP_PMCREAD system call on
all architectures. Some architectures offer faster methods of
reading these counts.
Sampling In sampling modes, the PMCs are configured to sample the CPU
instruction pointer (and optionally to capture the call chain
leading up to the sampled instruction pointer) after a config-
urable number of hardware events have been observed. Instruc-
tion pointer samples and call chain records are usually
directed to a log file for subsequent analysis.
Scope and operational mode are orthogonal; a PMC may thus be configured
to operate in one of the following four modes:
Process-scope, counting
These PMCs count hardware events whenever a thread in their
attached process is scheduled on a CPU. These PMCs normally
count from zero, but the initial count may be set using the
PMC_OP_SETCOUNT operation. Applications can read the value of
the PMC anytime using the PMC_OP_PMCRW operation.
Process-scope, sampling
These PMCs sample the target processes instruction pointer after
they have seen the configured number of hardware events. The
PMCs only count events when a thread belonging to their attached
process is active. The desired frequency of sampling is set
using the PMC_OP_SETCOUNT operation prior to starting the PMC.
Log files are configured using the PMC_OP_CONFIGURELOG operation.
System-scope, counting
These PMCs count hardware events seen by them independent of the
processes that are executing. The current count on these PMCs
can be read using the PMC_OP_PMCRW request. These PMCs normally
count from zero, but the initial count may be set using the
PMC_OP_SETCOUNT operation.
System-scope, sampling
These PMCs will periodically sample the instruction pointer of
the CPU they are allocated on, and will write the sample to a log
for further processing. The desired frequency of sampling is set
using the PMC_OP_SETCOUNT operation prior to starting the PMC.
Log files are configured using the PMC_OP_CONFIGURELOG operation.
System-wide statistical sampling can only be enabled by a process
with super-user privileges.
Processes are allowed to allocate as many PMCs as the hardware and cur-
rent operating conditions permit. Processes may mix allocations of sys-
tem-wide and process-private PMCs. Multiple processes may be using PMCs
simultaneously.
Allocated PMCs are started using the PMC_OP_PMCSTART operation, and
stopped using the PMC_OP_PMCSTOP operation. Stopping and starting a PMC
is permitted at any time the owner process has a valid handle to the PMC.
Process-private PMCs need to be attached to a target process before they
can be used. Attaching a process to a PMC is done using the
PMC_OP_PMCATTACH operation. An already attached PMC may be detached from
its target process using the converse PMC_OP_PMCDETACH operation. Issu-
ing a PMC_OP_PMCSTART operation on an as yet unattached PMC will cause it
to be attached to its owner process. The following rules determine
whether a given process may attach a PMC to another target process:
o A non-jailed process with super-user privileges is allowed to attach
to any other process in the system.
o Other processes are only allowed to attach to targets that they would
be able to attach to for debugging (as determined by p_candebug(9)).
PMCs are released using PMC_OP_PMCRELEASE. After a successful
PMC_OP_PMCRELEASE operation the handle to the PMC will become invalid.
Modifier Flags
The PMC_OP_PMCALLOCATE operation supports the following flags that modify
the behavior of an allocated PMC:
PMC_F_CALLCHAIN
This modifier informs sampling PMCs to record a callchain when
capturing a sample. The maximum depth to which call chains are
recorded is specified by the kern.hwpmc.callchaindepth kernel
tunable.
PMC_F_DESCENDANTS
This modifier is valid only for a PMC being allocated in process-
private mode. It signifies that the PMC will track hardware
events for its target process and the target's current and future
descendants.
PMC_F_KGMON
This modifier is valid only for a PMC being allocated in system-
wide sampling mode. It signifies that the PMC's sampling inter-
rupt is to be used to drive kernel profiling via kgmon(8). This
functionality is currently unimplemented.
PMC_F_LOG_PROCCSW
This modifier is valid only for a PMC being allocated in process-
private mode. When this modifier is present, at every context
switch, hwpmc will log a record containing the number of hardware
events seen by the target process when it was scheduled on the
CPU.
PMC_F_LOG_PROCEXIT
This modifier is valid only for a PMC being allocated in process-
private mode. With this modifier present, hwpmc will maintain
per-process counts for each target process attached to a PMC. At
process exit time, a record containing the target process' PID
and the accumulated per-process count for that process will be
written to the configured log file.
Modifiers PMC_F_LOG_PROCEXIT and PMC_F_LOG_PROCCSW may be used in combi-
nation with modifier PMC_F_DESCENDANTS to track the behavior of complex
pipelines of processes. PMCs with modifiers PMC_F_LOG_PROCEXIT and
PMC_F_LOG_PROCCSW cannot be started until their owner process has config-
ured a log file.
Signals
The hwpmc driver may deliver signals to processes that have allocated
PMCs:
SIGIO A PMC_OP_PMCRW operation was attempted on a process-private PMC
that does not have attached target processes.
SIGBUS The hwpmc driver is being unloaded from the kernel.
PMC ROW DISPOSITIONS
A PMC row is defined as the set of PMC resources at the same hardware
address in the CPUs in a system. Since process scope PMCs need to move
between CPUs following their target threads, allocation of a process
scope PMC reserves all PMCs in a PMC row for use only with process scope
PMCs. Accordingly a PMC row will be in one of the following disposi-
tions:
PMC_DISP_FREE Hardware counters in this row are free and may be
use to satisfy either of system scope or process
scope allocation requests.
PMC_DISP_THREAD Hardware counters in this row are in use by process
scope PMCs and are only available for process scope
allocation requests.
PMC_DISP_STANDALONE Some hardware counters in this row have been admin-
istratively disabled or are in use by system scope
PMCs. Non-disabled hardware counters in such a row
may be used for satisfying system scope allocation
requests. No process scope PMCs will use hardware
counters in this row.
[править] PROGRAMMING API
The recommended way for application programs to use the facilities of the
hwpmc driver is using the API provided by the pmc(3) library.
The hwpmc driver operates using a system call number that is dynamically
allotted to it when it is loaded into the kernel.
The hwpmc driver supports the following operations:
PMC_OP_CONFIGURELOG
Configure a log file for PMCs that require a log file. The hwpmc
driver will write log data to this file asynchronously. If it
encounters an error, logging will be stopped and the error code
encountered will be saved for subsequent retrieval by a
PMC_OP_FLUSHLOG request.
PMC_OP_FLUSHLOG
Transfer buffered log data inside hwpmc to a configured output
file. This operation returns to the caller after the write oper-
ation has returned. The returned error code reflects any pending
error state inside hwpmc.
PMC_OP_GETCPUINFO
Retrieve information about the highest possible CPU number for
the system, and the number of hardware performance monitoring
counters available per CPU.
PMC_OP_GETDRIVERSTATS
Retrieve module statistics (for analyzing the behavior of hwpmc
itself).
PMC_OP_GETMODULEVERSION
Retrieve the version number of API.
PMC_OP_GETPMCINFO
Retrieve information about the current state of the PMCs on a
given CPU.
PMC_OP_PMCADMIN
Set the administrative state (i.e., whether enabled or disabled)
for the hardware PMCs managed by the hwpmc driver. The invoking
process needs to possess the PRIV_PMC_MANAGE privilege.
PMC_OP_PMCALLOCATE
Allocate and configure a PMC. On successful allocation, a handle
to the PMC (a 32 bit value) is returned.
PMC_OP_PMCATTACH
Attach a process mode PMC to a target process. The PMC will be
active whenever a thread in the target process is scheduled on a
CPU.
If the PMC_F_DESCENDANTS flag had been specified at PMC alloca-
tion time, then the PMC is attached to all current and future
descendants of the target process.
PMC_OP_PMCDETACH
Detach a PMC from its target process.
PMC_OP_PMCRELEASE
Release a PMC.
PMC_OP_PMCRW
Read and write a PMC. This operation is valid only for PMCs con-
figured in counting modes.
PMC_OP_SETCOUNT
Set the initial count (for counting mode PMCs) or the desired
sampling rate (for sampling mode PMCs).
PMC_OP_PMCSTART
Start a PMC.
PMC_OP_PMCSTOP
Stop a PMC.
PMC_OP_WRITELOG
Insert a timestamped user record into the log file.
i386 Specific API
Some i386 family CPUs support the RDPMC instruction which allows a user
process to read a PMC value without needing to invoke a PMC_OP_PMCRW
operation. On such CPUs, the machine address associated with an allo-
cated PMC is retrievable using the PMC_OP_PMCX86GETMSR system call.
PMC_OP_PMCX86GETMSR
Retrieve the MSR (machine specific register) number associated
with the given PMC handle.
The PMC needs to be in process-private mode and allocated without
the PMC_F_DESCENDANTS modifier flag, and should be attached only
to its owner process at the time of the call.
amd64 Specific API
AMD64 CPUs support the RDPMC instruction which allows a user process to
read a PMC value without needing to invoke a PMC_OP_PMCRW operation. The
machine address associated with an allocated PMC is retrievable using the
PMC_OP_PMCX86GETMSR system call.
PMC_OP_PMCX86GETMSR
Retrieve the MSR (machine specific register) number associated
with the given PMC handle.
The PMC needs to be in process-private mode and allocated without
the PMC_F_DESCENDANTS modifier flag, and should be attached only
to its owner process at the time of the call.
[править] SYSCTL VARIABLES AND LOADER TUNABLES
The behavior of hwpmc is influenced by the following sysctl(8) and
loader(8) tunables:
kern.hwpmc.callchaindepth (integer, read-only)
The maximum number of call chain records to capture per sample.
The default is 8.
kern.hwpmc.debugflags (string, read-write)
(Only available if the hwpmc driver was compiled with -DDEBUG.)
Control the verbosity of debug messages from the hwpmc driver.
kern.hwpmc.hashsize (integer, read-only)
The number of rows in the hash tables used to keep track of owner
and target processes. The default is 16.
kern.hwpmc.logbuffersize (integer, read-only)
The size in kilobytes of each log buffer used by hwpmc's logging
function. The default buffer size is 4KB.
kern.hwpmc.mtxpoolsize (integer, read-only)
The size of the spin mutex pool used by the PMC driver. The
default is 32.
kern.hwpmc.nbuffers (integer, read-only)
The number of log buffers used by hwpmc for logging. The default
is 16.
kern.hwpmc.nsamples (integer, read-only)
The number of entries in the per-CPU ring buffer used during sam-
pling. The default is 16.
security.bsd.unprivileged_syspmcs (boolean, read-write)
If set to non-zero, allow unprivileged processes to allocate sys-
tem-wide PMCs. The default value is 0.
security.bsd.unprivileged_proc_debug (boolean, read-write)
If set to 0, the hwpmc driver will only allow privileged pro-
cesses to attach PMCs to other processes.
These variables may be set in the kernel environment using kenv(1) before
hwpmc is loaded.
[править] IMPLEMENTATION NOTES
SMP Symmetry
The kernel driver requires all physical CPUs in an SMP system to have
identical performance monitoring counter hardware.
Sparse CPU Numbering
On platforms that sparsely number CPUs and which support hot-plugging of
CPUs, requests that specify non-existent or disabled CPUs will fail with
an error. Applications allocating system-scope PMCs need to be aware of
the possibility of such transient failures.
x86 TSC Handling
Historically, on the x86 architecture, FreeBSD has permitted user pro-
cesses running at a processor CPL of 3 to read the TSC using the RDTSC
instruction. The hwpmc driver preserves this behavior.
Intel P4/HTT Handling
On CPUs with HTT support, Intel P4 PMCs are capable of qualifying only a
subset of hardware events on a per-logical CPU basis. Consequently, if
HTT is enabled on a system with Intel Pentium P4 PMCs, then the hwpmc
driver will reject allocation requests for process-private PMCs that
request counting of hardware events that cannot be counted separately for
each logical CPU.
Intel Pentium-Pro Handling
Writing a value to the PMC MSRs found in Intel Pentium-Pro style PMCs
(found in Intel Pentium Pro, Pentium II, Pentium III, Pentium M and
Celeron processors) will replicate bit 31 of the value being written into
the upper 8 bits of the MSR, bringing down the usable width of these PMCs
to 31 bits. For process-virtual PMCs, the hwpmc driver implements a
workaround in software and makes the corrected 64 bit count available via
the PMC_OP_RW operation. Processes that intend to use RDPMC instructions
directly or that intend to write values larger than 2^31 into these PMCs
with PMC_OP_RW need to be aware of this hardware limitation.
[править] DIAGNOSTICS
hwpmc: [class/npmc/capabilities]... Announce the presence of npmc PMCs
of class class, with capabilities described by bit string capabilities.
hwpmc: kernel version (0x%x) does not match module version (0x%x). The
module loading process failed because a version mismatch was detected
between the currently executing kernel and the module being loaded.
hwpmc: this kernel has not been compiled with 'options HWPMC_HOOKS'. The
module loading process failed because the currently executing kernel was
not configured with the required configuration option HWPMC_HOOKS.
hwpmc: tunable hashsize=%d must be greater than zero. A negative value
was supplied for tunable kern.hwpmc.hashsize.
hwpmc: tunable logbuffersize=%d must be greater than zero. A negative
value was supplied for tunable kern.hwpmc.logbuffersize.
hwpmc: tunable nlogbuffers=%d must be greater than zero. A negative
value was supplied for tunable kern.hwpmc.nlogbuffers.
hwpmc: tunable nsamples=%d out of range. The value for tunable
kern.hwpmc.nsamples was negative or greater than 65535.
[править] COMPATIBILITY
The hwpmc driver is currently under development. The API and ABI docu-
mented in this manual page may change in the future. The recommended
method of accessing this driver is using the pmc(3) API.
[править] ERRORS
A command issued to the hwpmc driver may fail with the following errors:
[EAGAIN] Helper process creation failed for a
PMC_OP_CONFIGURELOG request due to a temporary
resource shortage in the kernel.
[EBUSY] A PMC_OP_CONFIGURELOG operation was requested while an
existing log was active.
[EBUSY] A DISABLE operation was requested using the
PMC_OP_PMCADMIN request for a set of hardware
resources currently in use for process-private PMCs.
[EBUSY] A PMC_OP_PMCADMIN operation was requested on an active
system mode PMC.
[EBUSY] A PMC_OP_PMCATTACH operation was requested for a tar-
get process that already had another PMC using the
same hardware resources attached to it.
[EBUSY] A PMC_OP_PMCRW request writing a new value was issued
on a PMC that was active.
[EBUSY] A PMC_OP_PMCSETCOUNT request was issued on a PMC that
was active.
[EDOOFUS] A PMC_OP_PMCSTART operation was requested without a
log file being configured for a PMC allocated with
PMC_F_LOG_PROCCSW and PMC_F_LOG_PROCEXIT modifiers.
[EDOOFUS] A PMC_OP_PMCSTART operation was requested on a system-
wide sampling PMC without a log file being configured.
[EEXIST] A PMC_OP_PMCATTACH request was reissued for a target
process that already is the target of this PMC.
[EFAULT] A bad address was passed in to the driver.
[EINVAL] An invalid PMC handle was specified.
[EINVAL] An invalid CPU number was passed in for a
PMC_OP_GETPMCINFO operation.
[EINVAL] A PMC_OP_CONFIGURELOG request to de-configure a log
file was issued without a log file being configured.
[EINVAL] A PMC_OP_FLUSHLOG request was issued without a log
file being configured.
[EINVAL] An invalid CPU number was passed in for a
PMC_OP_PMCADMIN operation.
[EINVAL] An invalid operation request was passed in for a
PMC_OP_PMCADMIN operation.
[EINVAL] An invalid PMC ID was passed in for a PMC_OP_PMCADMIN
operation.
[EINVAL] A suitable PMC matching the parameters passed in to a
PMC_OP_PMCALLOCATE request could not be allocated.
[EINVAL] An invalid PMC mode was requested during a
PMC_OP_PMCALLOCATE request.
[EINVAL] An invalid CPU number was specified during a
PMC_OP_PMCALLOCATE request.
[EINVAL] A CPU other than PMC_CPU_ANY was specified in a
PMC_OP_PMCALLOCATE request for a process-private PMC.
[EINVAL] A CPU number of PMC_CPU_ANY was specified in a
PMC_OP_PMCALLOCATE request for a system-wide PMC.
[EINVAL] The pm_flags argument to an PMC_OP_PMCALLOCATE request
contained unknown flags.
[EINVAL] (On Intel Pentium 4 CPUs with HTT support) A
PMC_OP_PMCALLOCATE request for a process-private PMC
was issued for an event that does not support counting
on a per-logical CPU basis.
[EINVAL] A PMC allocated for system-wide operation was speci-
fied with a PMC_OP_PMCATTACH or PMC_OP_PMCDETACH
request.
[EINVAL] The pm_pid argument to a PMC_OP_PMCATTACH or
PMC_OP_PMCDETACH request specified an illegal process
ID.
[EINVAL] A PMC_OP_PMCDETACH request was issued for a PMC not
attached to the target process.
[EINVAL] Argument pm_flags to a PMC_OP_PMCRW request contained
illegal flags.
[EINVAL] A PMC_OP_PMCX86GETMSR operation was requested for a
PMC not in process-virtual mode, or for a PMC that is
not solely attached to its owner process, or for a PMC
that was allocated with flag PMC_F_DESCENDANTS.
[EINVAL] A PMC_OP_WRITELOG request was issued for an owner
process without a log file configured.
[ENOMEM] The system was not able to allocate kernel memory.
[ENOSYS] (On i386 and amd64 architectures) A
PMC_OP_PMCX86GETMSR operation was requested for hard-
ware that does not support reading PMCs directly with
the RDPMC instruction.
[ENXIO] A PMC_OP_GETPMCINFO operation was requested for an
absent or disabled CPU.
[ENXIO] A PMC_OP_PMCALLOCATE operation specified allocation of
a system-wide PMC on an absent or disabled CPU.
[ENXIO] A PMC_OP_PMCSTART or PMC_OP_PMCSTOP request was issued
for a system-wide PMC that was allocated on a CPU that
is currently absent or disabled.
[EOPNOTSUPP] A PMC_OP_PMCALLOCATE request was issued for PMC capa-
bilities not supported by the specified PMC class.
[EOPNOTSUPP] (i386 architectures) A sampling mode PMC was requested
on a CPU lacking an APIC.
[EPERM] A PMC_OP_PMCADMIN request was issued by a process
without super-user privilege or by a jailed super-user
process.
[EPERM] A PMC_OP_PMCATTACH operation was issued for a target
process that the current process does not have permis-
sion to attach to.
[EPERM] (i386 and amd64 architectures) A PMC_OP_PMCATTACH
operation was issued on a PMC whose MSR has been
retrieved using PMC_OP_PMCX86GETMSR.
[ESRCH] A process issued a PMC operation request without hav-
ing allocated any PMCs.
[ESRCH] A process issued a PMC operation request after the PMC
was detached from all of its target processes.
[ESRCH] A PMC_OP_PMCATTACH or PMC_OP_PMCDETACH request speci-
fied a non-existent process ID.
[ESRCH] The target process for a PMC_OP_PMCDETACH operation is
not being monitored by hwpmc.
[править] SEE ALSO
kenv(1), pmc(3), pmclog(3), kgmon(8), kldload(8), pmccontrol(8), pmcstat(8), sysctl(8), kproc_create(9), p_candebug(9)
[править] HISTORY
The hwpmc driver first appeared in FreeBSD 6.0.
[править] AUTHORS
The hwpmc driver was written by Joseph Koshy <jkoshy@FreeBSD.org>.
[править] BUGS
The driver samples the state of the kernel's logical processor support at
the time of initialization (i.e., at module load time). On CPUs support-
ing logical processors, the driver could misbehave if logical processors
are subsequently enabled or disabled while the driver is active.
On the i386 architecture, the driver requires that the local APIC on the
CPU be enabled for sampling mode to be supported. Many single-processor
motherboards keep the APIC disabled in BIOS; on such systems hwpmc will
not support sampling PMCs.
[править] SECURITY CONSIDERATIONS
PMCs may be used to monitor the actual behavior of the system on hard-
ware. In situations where this constitutes an undesirable information
leak, the following options are available:
1. Set the sysctl(8) tunable security.bsd.unprivileged_syspmcs to 0.
This ensures that unprivileged processes cannot allocate system-wide
PMCs and thus cannot observe the hardware behavior of the system as
a whole. This tunable may also be set at boot time using loader(8),
or with kenv(1) prior to loading the hwpmc driver into the kernel.
2. Set the sysctl(8) tunable security.bsd.unprivileged_proc_debug to 0.
This will ensure that an unprivileged process cannot attach a PMC to
any process other than itself and thus cannot observe the hardware
behavior of other processes with the same credentials.
System administrators should note that on IA-32 platforms FreeBSD makes
the content of the IA-32 TSC counter available to all processes via the
RDTSC instruction.
FreeBSD 9.0 September 22, 2008 FreeBSD 9.0
