PMCTrack is an open-source OS-oriented performance monitoring tool for GNU/Linux. This performance tool has been specifically designed to aid kernel developers in implementing scheduling algorithms in Linux that leverage data from performance monitoring counters (PMCs) to perform optimizations at run time. Unlike other monitoring tools, PMCTrack features and in-kernel API enabling the OS scheduler to access per-thread PMC data in an architecture-independent fashion.
Despite being an OS-oriented tool, PMCTrack still allows gathering PMC values from user space, enabling kernel developers to carry out the necessary offline analysis and debugging to assist them during the scheduler design process. In addition, the tool provides both the scheduler and the userspace PMCTrack components with other insightful metrics available in modern processors that are not directly exposed as PMCs, such as cache occupancy or energy consumption.
- Juan Carlos Saez Alcaide (firstname.lastname@example.org) - Creator of PMCTrack and main maintainer.
- Jorge Casas Hernan - Maintainer of PMCTrack-GUI
- Abel Serrano Juste (aka @Akronix)
- Javier Setoain
- Guillermo Martinez Fernandez
- Sergio Sanchez Gordo
- Sofia Dronda Merino
- Juan Carlos Saez, Adrian Pousa, Roberto Rodríguez-Rodríguez, Fernando Castro, Manuel Prieto-Matias. (2017) "PMCTrack: Delivering performance monitoring counter support to the OS scheduler", The Computer Journal, Volume 60, Issue 1, 1 January 2017, pp. 60–85. (pdf)
- Juan Carlos Saez, Jorge Casas, Abel Serrano, Roberto Rodríguez-Rodríguez, Fernando Castro, Daniel Chaver, Manuel Prieto-Matias. (2015) "An OS-Oriented Performance Monitoring Tool for Multicore Systems," Euro-Par 2015 International Workshops: Revised Selected Papers, pp. 697-709.
To support PMCTrack, a patched Linux kernel must be installed on the machine. A number of kernel patches for various Linux versions can be found in the
src/kernel-patches directory. The name of each patch file encodes the Linux kernel version where the patch must be applied to as well as the processor architecture supported. The format is as follows:
To build the kernel for PMCTrack, the following option must be enabled when configuring the kernel:
The kernel headers for the patched Linux version must be installed on the system as well. This is necessary for a successful out-of-tree build of PMCTrack's kernel module. An out-of-tree-ready Makefile can be found in the sources for the different flavors of the kernel module.
Most PMCTrack user-level components are written in C, and do not depend on any external library, (beyond the libc, of course). A separate Makefile is provided for libpmctrack as well as for the various command-line tools. As such, it should be straightforward to build these software components on most Linux distributions.
We also created PMCTrack-GUI, a Python front-end for the
pmctrack command-line tool. This application extends the capabilities of the PMCTrack stack with features such as an SSH-based remote monitoring mode or the ability to plot the values of user-defined performance metrics in real time. This GUI application runs on Linux and Mac OS X and has the following software dependencies:
- Python v2.7
- Matplotlib (Python library)
- sshpass (command)
- WxPython v3.0
On Debian or Ubuntu the necessary software can be installed as follows:
$ sudo apt-get install python2.7 python-matplotlib python-wxgtk3.0 sshpass
On Mac OS X, PMCTrack-GUI has been succesfully tested after installing the above software dependencies using MacPorts as follows:
## Install packages $ sudo port install py27-matplotlib py27-numpy py27-scipy py27-ipython py27-wxpython-3.0 sshpass ## Set up default configuration for matplotlib $ mkdir ~/.matplotlib $ cp /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/mpl-data/matplotlibrc ~/.matplotlib ## Select MacPorts Python27 interpreter by default $sudo port select --set python python27 $sudo port select --set ipython ipython27
Building PMCTrack from source for ARM and x86 processors
PMCTRACK_ROOT environment variable must be defined for a successful execution of the various PMCTrack command-line tools. The
shrc script found in the repository's root directory can be used to set the
PMCTRACK_ROOT variable appropriately as well as to add command-line tools' directories to the PATH. To make this possible, run the following command in the root directory of the repository:
$ . shrc
Now kernel-level and user-level components can be easily built with the
pmctrack-manager script as follows:
$ pmctrack-manager build **** System information *** Processor_vendor=Intel Kernel_HZ=250 Processor_bitwidth=64 *********************************************** Press ENTER to start the build process... ************************************************* *** Building supported PMCTrack kernel modules ** ************************************************* Building kernel module intel-core.... ============================================ make -C /lib/modules/3.17.3.pmctrack-x86+/build M=/home/bench/pmctrack/src/modules/pmcs/intel-core modules make: Entering directory '/usr/src/linux-headers-3.17.3.pmctrack-x86+' CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/mchw_core.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/mc_experiments.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/pmu_config_x86.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/cbuffer.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/monitoring_mod.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/syswide.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/intel_cmt_mm.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/intel_rapl_mm.o CC [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/ipc_sampling_sf_mm.o LD [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/mchw_intel_core.o Building modules, stage 2. MODPOST 1 modules CC /home/bench/pmctrack/src/modules/pmcs/intel-core/mchw_intel_core.mod.o LD [M] /home/bench/pmctrack/src/modules/pmcs/intel-core/mchw_intel_core.ko make: Leaving directory '/usr/src/linux-headers-3.17.3.pmctrack-x86+' Done!! ============================================ Building kernel module core2.... ============================================ make -C /lib/modules/3.17.3.pmctrack-x86+/build M=/home/bench/pmctrack/src/modules/pmcs/core2 modules make: Entering directory '/usr/src/linux-headers-3.17.3.pmctrack-x86+' CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/mchw_core.o CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/mc_experiments.o CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/pmu_config_x86.o /home/bench/pmctrack/src/modules/pmcs/core2/pmu_config_x86.c: In function 'init_pmu_props': /home/bench/pmctrack/src/modules/pmcs/core2/pmu_config_x86.c:155:6: warning: unused variable 'model_cpu' [-Wunused-variable] int model_cpu=0; ^ CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/cbuffer.o CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/monitoring_mod.o CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/syswide.o CC [M] /home/bench/pmctrack/src/modules/pmcs/core2/ipc_sampling_sf_mm.o LD [M] /home/bench/pmctrack/src/modules/pmcs/core2/mchw_core2.o Building modules, stage 2. MODPOST 1 modules CC /home/bench/pmctrack/src/modules/pmcs/core2/mchw_core2.mod.o LD [M] /home/bench/pmctrack/src/modules/pmcs/core2/mchw_core2.ko make: Leaving directory '/usr/src/linux-headers-3.17.3.pmctrack-x86+' Done!! ============================================ Building libpmctrack .... ============================================ make -C src all make: Entering directory '/home/bench/pmctrack/src/lib/libpmctrack/src' cc -c -DHZ=250 -Wall -g -fpic -I ../include -I ../../../modules/pmcs/include/pmc -o core.o core.c cc -c -DHZ=250 -Wall -g -fpic -I ../include -I ../../../modules/pmcs/include/pmc -o pmu_info.o pmu_info.c cc -shared -DHZ=250 -o ../libpmctrack.so core.o pmu_info.o ar rcs ../libpmctrack.a core.o pmu_info.o make: Leaving directory '/home/bench/pmctrack/src/lib/libpmctrack/src' Done!! ============================================ Building pmc-events .... ============================================ cc -DUSE_VFORK -Wall -g -I ../../modules/pmcs/include/pmc -I../../lib/libpmctrack/include -c -o pmc-events.o pmc-events.c cc -o ../../../bin/pmc-events pmc-events.o -L../../lib/libpmctrack -lpmctrack -static Done!! ============================================ Building pmctrack .... ============================================ cc -DUSE_VFORK -Wall -g -I ../../modules/pmcs/include/pmc -I../../lib/libpmctrack/include -c -o pmctrack.o pmctrack.c cc -o ../../../bin/pmctrack pmctrack.o -L../../lib/libpmctrack -lpmctrack -static Done!! ============================================ *** BUILD PROCESS COMPLETED SUCCESSFULLY ***
pmctrack-manager retrieves key information from the system and builds the command-line tools as well as the different flavors of the PMCTrack kernel module compatible with the current platform. If the build fails, build errors can be found in the
build.log file created in the current directory.
Building PMCTrack from source for the Intel Xeon Phi
In order to build the various PMCTrack components from source for the Xeon Phi Coprocessor
k1om-mpss-linux-gcc cross-compiler must be used. Such a compiler is bundled with Intel MPSS. For a successful compilation,
pmctrack-manager has to know the location of the Intel MPSS installation in the file system. In addition, the out-of-tree compilation of PMCTrack's kernel module for the Xeon Phi requires the build to be performed against a freshly built Linux kernel tree (MPSS version) with the PMCTrack patch. The entire source kernel tree must be found in the same system where the PMCTrack compilation is performed.
Once these requirements are met,
pmctrack-manager can be used as follows to perform the build for the Xeon Phi:
$ pmctrack-manager build-phi <mpss-root-dir> <kernel-sources-dir>
Using PMCTrack from user space
Once the build has been completed, go to the root directory of you local copy of PMCTrack's repository and set up the necessary environment variables as follows:
$ . shrc
After that, load any of the available flavors of the PMCTrack's kernel module compatible with your processor. Note that builds performed with
pmctrack-manager (as shown above), only compile those flavors of the kernel module that may be suitable for your system.
The following table summarizes the properties of the various flavors of the kernel module:
|Name||Path of the .ko file||Supported processors|
||Most Intel multi-core processors are compatible with this module, including recent processors based on the Intel "Broadwell" microarchitecture.|
||This module has been successfully tested on AMD opteron processors. Nevertheless, it should be compatible with all AMD multicore processors.|
||This module has been successfully tested on ARM systems featuring 32-bit big.LITTLE processors, which combine ARM Cortex A7 cores with and ARM Cortex A15 cores. Specifically, tests were performed on the ARM Coretile Express Development Board (TC2).|
||Specific module for Odroid XU3 and XU4 boards. More information on these boards can be found at www.hardkernel.com|
||This module has been successfully tested on ARM systems featuring 64-bit big.LITTLE processors, which combine ARM Cortex A57 cores with and ARM Cortex A53 cores. Specifically, tests were performed on the ARM Juno Development Board.|
||Intel Xeon Phi Coprocessor|
||This module has been specifically designed for the Intel QuickIA prototype system. The Intel QuickIA is a dual-socket asymmetric multicore system that features a quad-core Intel Xeon E5450 processor and a dual-core Intel Atom N330. The module also works with Intel Atom processors as well as "old" Intel multicore processors, such as the Intel Core 2 Duo. Nevertheless, given the numerous existing hacks for the QuickIA in this module, users are advised to use the more general "intel-core" flavor.|
Once the most suitable kernel model for the system has been identified, the module can be loaded in the running PMCTrack-enabled kernel as follows:
$ sudo insmod <path_to_the_ko_file>
If the command did not return errors, information on the detected Performance Monitoring Units (PMUs) found in the machine can be retrieved by reading from the
$ cat /proc/pmc/info *** PMU Info *** nr_core_types=1 [PMU coretype0] pmu_model=x86_intel-core.haswell-ep nr_gp_pmcs=8 nr_ff_pmcs=3 pmc_bitwidth=48 *************** *** Monitoring Module *** counter_used_mask=0x0 nr_experiments=0 nr_virtual_counters=0 ***************
pmc-events helper command can be used to get the same information in a slightly different format:
$ pmc-events -I [PMU 0] pmu_model=x86_intel-core.haswell-ep nr_fixed_pmcs=3 nr_gp_pmcs=8
On systems featuring asymmetric multicore processors, such as the ARM big.LITTLE, the
pmc-event command will list as many PMUs as different core types exist in the system. On a 64-bit ARM big.LITTLE processor the output will be as follows:
$ pmc-events -I [PMU 0] pmu_model=armv8.cortex_a53 nr_fixed_pmcs=1 nr_gp_pmcs=6 [PMU 1] pmu_model=armv8.cortex_a57 nr_fixed_pmcs=1 nr_gp_pmcs=6
To obtain a listing of the hardware events supported, the following command can be used:
$ pmc-events -L [PMU 0] instr_retired_fixed unhalted_core_cycles_fixed unhalted_ref_cycles_fixed instr cycles unhalted_core_cycles instr_retired unhalted_ref_cycles llc_references llc_references.prefetch llc_misses llc_misses.prefetch branch_instr_retired branch_mispred_retired l2_references l2_misses ...
pmctrack command-line tool
pmctrack command is the most straigthforward way of accessing PMCTrack functionality from user space. The available options for the command can be listed by just typing
pmctrack in the console:
$ pmctrack Usage: pmctrack [OPTION [OP. ARGS]] [PROG [ARGS]] Available options: -c <config-string> set up a performance monitoring experiment using either raw or mnemonic-based PMC string -o <output> output: set output file for the results. (default = stdout.) -T <Time> Time: elapsed time in seconds between two consecutive counter samplings. (default = 1 sec.) -b <cpu or mask> bind launched program to the specified cpu o cpumask. -n <max-samples> Run command until a given number of samples are collected -N <secs> Run command for secs seconds only -e Enable extended output -A Enable aggregate count mode -k <kernel_buffer_size> Specify the size of the kernel buffer used for the PMC samples -b <cpu or mask> bind monitor program to the specified cpu o cpumask. -S Enable system-wide monitoring mode (per-CPU) -r Accept pmc configuration strings in the RAW format -p <pmu> Specify the PMU id to use for the event configuration -L Legacy-mode: do not show counter-to-event mapping -t Show real, user and sys time of child process PROG + ARGS: Command line for the program to be monitored.
Before introducing the basics of this command, it is worth describing the semantics of the
-V options. Essentially, the -c option accepts an argument with a string describing a set of hardware events to be monitored. This string consists of comma-separated event configurations; in turn, an event configuration can be specified using event mnemonics or event hex codes found in the PMU manual provided by the processor's manufacturer. For example the hex-code based string
0x8,0x11 for an ARM Cortex A57 processor specifies the same event set than that of the
instr,cycles string. Clearly, the latter format is far more intuitive than the former; the user can probably guess that we are trying to specify the hardware events "retired instructions" and "cycles".
-V option makes it possible to specify a set of virtual counters. Modern systems enable monitoring a set of hardware events using PMCs. Still, other monitoring information (e.g., energy consumption) may be exposed to the OS by other means, such as fixed-function registers, sensors, etc. This "non-PMC" data is exposed by the PMCTrack kernel module as virtual counters, rather than HW events. PMCTrack monitoring modules are in charge of implementing low-level access to virtual counters. To retrieve the list of HW events exported by the active monitoring module use
pmc-events -V. More information on PMCTrack monitoring modules can be found in a separate section of this document.
pmctrack command supports three usage modes:
- Time-Based Sampling (TBS): PMC and virtual counter values for a certain application are collected at regular time intervals.
- Event-Based Sampling (EBS): PMC and virtual counter values for an application are collected every time a given PMC event reaches a given count.
- Time-Based system-wide monitoring mode: This mode is a variant of the TBS mode, but monitoring information is provided for each CPU in the system, rather than for a specific application. This mode can be enabled with the
To illustrate how the TBS mode works let us consider the following example command invoked on a system featuring a quad-core Intel Xeon Haswell processor:
$ pmctrack -T 1 -c instr,llc_misses -V energy_core ./mcf06 [Event-to-counter mappings] pmc0=instr pmc3=llc_misses virt0=energy_core [Event counts] nsample pid event pmc0 pmc3 virt0 1 10767 tick 2017968202 30215772 7930969 2 10767 tick 1220639346 24866936 7580993 3 10767 tick 1204660012 24726068 7432617 4 10767 tick 1524589394 20013147 8411560 5 10767 tick 1655802083 9520886 8531860 6 10767 tick 2555712483 18420142 6615844 7 10767 tick 2222232510 19594864 6385986 8 10767 tick 1348937378 22795510 5966308 9 10767 tick 1455948820 22282935 5994934 10 10767 tick 1324007762 22682355 5951354 11 10767 tick 1345928005 22477525 5968872 12 10767 tick 1345868008 22400733 6024780 13 10767 tick 1370194276 22121318 6024169 14 10767 tick 1329712408 22154371 6030700 15 10767 tick 1365130132 21859147 6076293 16 10767 tick 1315829803 21780616 6136962 17 10767 tick 1357349957 20889360 6234619 18 10767 tick 1377910047 19539232 6519897 ...
This command provides the user with the number of instructions retired, last-level cache (LLC) misses and core energy consumption (in uJ) every second. The beginning of the command output shows the event-to-counter mapping for the various hardware events and virtual counters. The "Event counts" section in the output displays a table with the raw counts for the various events; each sample (one per second) is represented by a different row. Note that the sampling period is specified in seconds via the -T option; fractions of a second can be also specified (e.g, 0.3 for 300ms). If the user includes the -A switch in the command line,
pmctrack will display the aggregate event count for the application's entire execution instead. At the end of the line, we specify the command to run the associated application we wish to monitor (e.g: ./mcf06).
In case a specific processor model does not integrate enough PMCs to monitor a given set of events at once, the user can turn to PMCTrack's event-multiplexing feature. This boils down to specifying several event sets by including multiple instances of the -c switch in the command line. In this case, the various events sets will be collected in a round-robin fashion and a new
expid field in the output will indicate the event set a particular sample belongs to. In a similar vein, time-based sampling also supports multithreaded applications. In this case, samples from each thread in the application will be identified by a different value in the pid column.
Event-based Sampling (EBS) constitutes a variant of time-based sampling wherein PMC values are gathered when a certain event count reaches a certain threshold T. To support EBS, PMCTrack's kernel module exploits the interrupt-on-overflow feature present in most modern Performance Monitoring Units (PMUs). To use the EBS feature from userspace, the "ebs" flag must be specified in
pmctrack command line by an event's name. In doing so, a threshold value may be also specified as in the following example:
$ pmctrack -c instr:ebs=500000000,llc_misses -V energy_core ./mcf06 [Event-to-counter mappings] pmc0=instr pmc3=llc_misses virt0=energy_core [Event counts] nsample pid event pmc0 pmc3 virt0 1 10839 ebs 500000078 892837 526489 2 10839 ebs 500000047 9383500 1946166 3 10839 ebs 500000050 9692922 2544250 4 10839 ebs 500000007 10017122 2818298 5 10839 ebs 500000012 9907918 3055236 6 10839 ebs 500000011 10335579 3108215 7 10839 ebs 500000046 10735151 3118713 8 10839 ebs 500000011 10335980 3119140 9 10839 ebs 500000004 10250777 3053100 10 10839 ebs 500000019 11382679 2997802 11 10839 ebs 500000035 6650139 2587890 12 10839 ebs 500000004 474847 2596313 13 10839 ebs 500000039 532301 2601074 14 10839 ebs 500000019 577618 2617187 15 10839 ebs 500000062 6221112 2442504 16 10839 ebs 500000037 9177684 2325317 17 10839 ebs 500000058 2697348 1106994 18 10839 ebs 500000006 3520781 1264404 19 10839 ebs 500000055 2777145 1119934 20 10839 ebs 500000034 1965457 964843 21 10839 ebs 500000004 2290861 1035095 22 10839 ebs 500000011 3276917 1217895 23 10839 ebs 500000041 4202958 1409973 24 10839 ebs 500000034 5343461 1608947 ...
virt0 columns display the number of LLC misses and energy consumption every 500 million retired instructions. Note, however, that values in the
pmc0 column do not reflect exactly the target instruction count. This has to do with the fact that, in modern processors, the PMU interrupt is not served right after the counter overflows. Instead, due to the out-of-order and speculative execution, several dozen instructions or more may be executed within the period elapsed from counter overflow until the application is actually interrupted. These inaccuracies do not pose a big problem as long as coarse instruction windows are used.
Another way of accessing PMCTrack functionality from user space is via libpmctrack. This library enables to characterize performance of specific code fragments via PMCs and virtual counters in sequential and multithreaded programs written in C or C++. Libpmctrack's API makes it possible to indicate the desired PMC and virtual-counter configuration to the PMCTrack's kernel module at any point in the application's code or within a runtime system. The programmer may then retrieve the associated event counts for any code snippet (via TBS or EBS) simply by enclosing the code between invocations to the
pmctrack_stop_count() functions. To illustrate the use of libpmctrack, several example programs are provided in the repository under
PMCTrack monitoring modules
PMCTrack's kernel module can be easily extended with support for extra HW monitoring facilities not implemented in the basic PMCTrack stack. To implement such an extension a new PMCTrack monitoring module must be implemented. Several sample monitoring modules are provided along with the PMCTrack distribution; its source code can be found in the
*_mm.c files found in
From the programmer's standpoint, creating a monitoring module entails implementing the
monitoring_module_t interface (
<pmc/monitoring_mod.h>) in a separate .c file of the PMCTrack kernel module sources. The
monitoring_module_t interface consists of several callback functions enabling to notify the module on activations/deactivations requested by the system administrator, on threads' context switches, every time a thread enters/exits the system, etc. The programmer typically implements only the subset of callbacks required to carry out the necessary internal processing. Notably, any kind of monitoring information accessed by the monitoring module can be exposed to PMCTrack userland tools as a virtual counter.
PMCTrack's kernel module also enables monitoring modules to take full control of performance monitoring counters to perform any kind of internal task. To make this possible, the monitoring module's developer does not have to deal with performance-counter registers directly. Instead, the programmer indicates the desired counter configuration (encoded in a string) using an API function. Whenever new PMC samples are collected for a thread, a callback function of the monitoring module gets invoked, passing the samples as a parameter. Thanks to this feature, a monitoring module will only access low-level registers to provide the scheduler or the end user with other hardware monitoring information not modeled as PMC events, such as temperature or energy consumption.
PMCTrack may include several monitoring modules compatible with a given platform. However, only one can be enabled at a time. Monitoring modules available for the current system can be obtained by reading from the
$ cat /proc/pmc/mm_manager [*] 0 - This is just a proof of concept [ ] 1 - IPC sampling-based SF estimation model [ ] 2 - PMCtrack module that supports Intel CMT [ ] 3 - PMCtrack module that supports Intel RAPL
In the example above, four monitoring modules are listed and module #0, marked with "*", is the active monitoring module.
In the event several compatible monitoring modules exist, the system administrator may tell the system which one to use by writing in the
/proc/pmc/mm_manager file as follows:
$ echo 'activate 3' > /proc/pmc/mm_manager $ cat /proc/pmc/mm_manager [ ] 0 - This is just a proof of concept [ ] 1 - IPC sampling-based SF estimation model [ ] 2 - PMCtrack module that supports Intel CMT [*] 3 - PMCtrack module that supports Intel RAPL
pmc-events command can be used to list the virtual counters exported by the active monitoring module, as follows:
$ pmc-events -V [Virtual counters] energy_pkg energy_dram
Using PMCTrack from the OS scheduler
PMCTrack allows any scheduling algorithm in the Linux kernel (i.e., scheduling class) to collect per-thread monitoring data, thus making it possible to drive scheduling decisions based on tasks' memory behavior or other microarchitectural properties. Turning on this mode for a particular thread from the scheduler's code boils down to activating the
prof_enabled flag in the thread's descriptor. This flag is added to Linux's task structure when applying PMCTrack's kernel patch.
To ensure that the implementation of the scheduling algorithm that benefits from this feature remains architecture independent, the scheduler itself (implemented in the kernel) does not configure nor deals with performance counters directly. Instead, the active monitoring module in PMCTrack is in charge of feeding the scheduling policy with the necessary high-level performance monitoring metrics, such as a task's instruction per cycle ratio or its last-level-cache miss rate.
The scheduler can communicate with the active monitoring module to obtain per-thread data via the following function from PMCTrack's kernel API:
int pmcs_get_current_metric_value( struct task_struct* task, int metric_id, uint64_t* value );
For simplicity, each metric is assigned a numerical ID, known by the scheduler and the monitoring module. To obtain the up-to date value for a specific metric, the aforementioned function may be invoked from the tick processing function in the scheduler.
Monitoring modules make it possible for a scheduling policy relying on PMC or virtual counter metrics to be seamlessly extended to new architectures or processor models as long as the hardware enables to collect necessary monitoring data. All that needs to be done is to build a monitoring module or adapt an existing one to the platform in question.