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HWPMC(4) Kernel Interfaces Manual HWPMC(4) NAME hwpmc -- Hardware Performance Monitoring Counter support SYNOPSIS The following option must be present in the kernel configuration file: options HWPMC_HOOKS Additionally, for i386 systems: device apic To load the driver as a module at boot time: sysrc kld_list+=hwpmc Alternatively, to compile the driver into the kernel: device hwpmc To enable debugging features (see "DEBUGGING"): options KTR options KTR_COMPILE=(KTR_SUBSYS) options KTR_MASK=(KTR_SUBSYS) options HWPMC_DEBUG DESCRIPTION The hwpmc driver virtualizes the hardware performance monitoring facilities 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 ar- chitectures. Some architectures offer faster methods of reading these counts. Sampling In sampling modes, the PMCs are configured to sample the CPU in- struction pointer (and optionally to capture the call chain lead- ing up to the sampled instruction pointer) after a configurable number of hardware events have been observed. Instruction 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 at- tached 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 current operating conditions permit. Processes may mix allocations of system-wide and process-private PMCs. Multiple processes may be using PMCs simultane- ously. Allocated PMCs are started using the PMC_OP_PMCSTART operation, and stopped using the PMC_OP_PMCSTOP operation. Stopping and starting a PMC is permit- ted 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. Issuing 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: * A non-jailed process with super-user privileges is allowed to attach to any other process in the system. * 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 cap- turing a sample. The maximum depth to which call chains are recorded is specified by the kern.hwpmc.callchaindepth kernel tun- able. 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 descen- dants. 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 combina- tion 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 ad- dress in the CPUs in a system. Since process scope PMCs need to move be- tween 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 dispositions: PMC_DISP_FREE Hardware counters in this row are free and may be use to satisfy either of system scope or process scope al- location 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 adminis- tratively 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. COMPATIBILITY The API and ABI documented in this manual page may change in the future. This interface is intended to be consumed by the pmc(3) library; other con- sumers are unsupported. Applications targeting PMCs should use the pmc(3) library API. PROGRAMMING API 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 en- counters an error, logging will be stopped and the error code en- countered 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 opera- tion 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 it- self). 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 ac- tive whenever a thread in the target process is scheduled on a CPU. If the PMC_F_DESCENDANTS flag had been specified at PMC allocation time, then the PMC is attached to all current and future descen- dants 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 sam- pling 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 opera- tion. On such CPUs, 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. 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. 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 256KB. The maximum value is 16MB. kern.hwpmc.mincount (integer, read-write) The minimum sampling rate for sampling mode PMCs. The default count is 1000 events. kern.hwpmc.mtxpoolsize (integer, read-only) The size of the spin mutex pool used by the PMC driver. The de- fault is 32. kern.hwpmc.nbuffers_pcpu (integer, read-only) The number of log buffers per CPU used by hwpmc for logging. The default is 32. The product of kern.hwpmc.nbuffers_pcpu and kern.hwpmc.logbuffersize must not exceed 32MB per CPU. kern.hwpmc.nsamples (integer, read-only) The number of entries in the per-CPU ring buffer used during sam- pling. The default is 512. 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 processes to attach PMCs to other processes. These variables may be set in the kernel environment using kenv(1) before hwpmc is loaded. IMPLEMENTATION NOTES SMP Symmetry The kernel driver requires all physical CPUs in an SMP system to have iden- tical 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 processes running at a processor CPL of 3 to read the TSC using the RDTSC instruc- tion. 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 log- ical CPU. 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 be- tween 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: logbuffersize=%d must be greater than zero and less than or equal to %d, resetting to %d. A negative value was supplied for tunable kern.hwpmc.logbuffersize. hwpmc: nbuffers_pcpu=%d must be greater than zero, resetting to %d. A neg- ative value was supplied for tunable kern.hwpmc.nbuffers_pcpu. hwpmc: tunable nsamples=%d out of range. The value for tunable kern.hwpmc.nsamples was negative or greater than 65535. hwpmc: nbuffers_pcpu=%d * logbuffersize=%d exceeds %dMB per CPU limit, re- setting to defaults (%d * %d). The product of tunables kern.hwpmc.nbuffers_pcpu and kern.hwpmc.logbuffersize exceeds the maximum per-CPU memory limit. Both tunables are reset to their compiled defaults. DEBUGGING The hwpmc module can be configured to record trace entries using the ktr(4) interface. This is useful for debugging the driver's functionality, pri- marily during development. This debugging functionality is not enabled by default, and requires recompiling the kernel and hwpmc module after adding the following to the kernel config: options KTR options KTR_COMPILE=(KTR_SUBSYS) options KTR_MASK=(KTR_SUBSYS) options HWPMC_DEBUG This alone is not enough to enable tracing; one must also configure the kern.hwpmc.debugflags sysctl(8) variable, which provides fine-grained con- trol over which types of events are logged to the trace buffer. hwpmc trace events are grouped by 'major' and 'minor' flag types. The ma- jor flag names are as follows: cpu CPU events csw Context switch events logging Logging events md Machine-dependent/class-dependent events module Miscellaneous events owner PMC owner events pmc PMC management events process Process events sampling Sampling events The minor flags for each major flag group can vary. The individual minor flag names are: allocaterow, allocate, attach, bind, config, exec, exit, find, flush, fork, getbuf, hook, init, intr, linktarget, mayberemove, ops, read, register, release, remove, sample, scheduleio, select, signal, swi, swo, start, stop, syscall, unlinktarget, write The kern.hwpmc.debugflags variable is a string with a custom format. The string should contain a space-separated list of event specifiers. Each event specifier consists of the major flag name, followed by an equal sign (=), followed by a comma-separated list of minor event types. To track all events for a major group, an asterisk (*) can be given instead of minor event names. For example, to trace all allocation and release events, set debugflags as follows: kern.hwpmc.debugflags="pmc=allocate,release md=allocate,release" To trace all events in the process and context switch major flag groups: kern.hwpmc.debugflags="process=* csw=*" To disable all trace events, set the variable to an empty string. kern.hwpmc.debugflags="" Trace events are recorded by ktr(4), and can be inspected at run-time using the ktrdump(8) utility, or at the ddb(4) prompt after a panic with the 'show ktr' command. 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 ker- nel. [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 target 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] The pm_flags argument to a PMC_OP_CONFIGURELOG request contained unknown flags. [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 specified 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 at- tached 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 hardware that does not sup- port reading PMCs directly with the RDPMC instruction. [ENXIO] A PMC_OP_GETPMCINFO operation was requested for an ab- sent 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 capabil- ities 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 with- out 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 opera- tion was issued on a PMC whose MSR has been retrieved using PMC_OP_PMCX86GETMSR. [ESRCH] A process issued a PMC operation request without having 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 specified a non-existent process ID. [ESRCH] The target process for a PMC_OP_PMCDETACH operation is not being monitored by hwpmc. SEE ALSO kenv(1), pmc(3), pmclog(3), ddb(4), ktr(4), kldload(8), ktrdump(8), pmccon- trol(8), pmcstat(8), sysctl(8), kproc_create(9), p_candebug(9) HISTORY The hwpmc driver first appeared in FreeBSD 6.0. AUTHORS The hwpmc driver was written by Joseph Koshy <jkoshy@FreeBSD.org>. 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 supporting 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. SECURITY CONSIDERATIONS PMCs may be used to monitor the actual behavior of the system on hardware. 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 be- havior 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 ports 15.quarterly July 8, 2023 HWPMC(4)
NAME | SYNOPSIS | DESCRIPTION | COMPATIBILITY | PROGRAMMING API | SYSCTL VARIABLES AND LOADER TUNABLES | IMPLEMENTATION NOTES | DIAGNOSTICS | DEBUGGING | ERRORS | SEE ALSO | HISTORY | AUTHORS | BUGS | SECURITY CONSIDERATIONS
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