iwereth / libpfc

A small library and kernel module for easy access to x86 performance monitor counters under Linux.

libpfc

A small library and kernel module for easy access to x86 performance monitor counters under Linux.

Intro

libpfc is both a library and a Linux Loadable Kernel Module (LKM) that permits near-0-overhead use of the performance monitoring capabilities of modern x86 processors.

Getting the code, building and installing

libpfc's source code can be found on Github. It uses the Meson 0.41.0 build system, which itself requires Ninja, but itself has no dependencies. It can be build as follows.

    cd       /path/to/some/folder
    git      clone 'https://github.com/obilaniu/libpfc.git' libpfc
    mkdir    libpfc/build
    cd       libpfc/build
    meson.py ..  -Dbuildtype=release --prefix=/path/to/prefixdir  # Such as $HOME/.local
    ninja
    ninja    install

Make sure a path to libpfc.so is present in LD_LIBRARY_PATH.

Loading the kernel module pfc.ko

The kernel module is installed in /path/to/prefixdir/bin/pfc.ko. To load it into the kernel, ensure that no other entity is using the perf functionality, including other kernel modules such as NMI watchdogs. As of Linux 4.4.5, this can be done, with root privileges, as follows:

    modprobe -ar iTCO_wdt iTCO_vendor_support
    echo 0 > /proc/sys/kernel/nmi_watchdog
    echo 2 > /sys/bus/event_source/devices/cpu/rdpmc
    insmod /path/to/prefixdir/bin/pfc.ko

Using libpfc in user-space

Include

#include "libpfc.h"

This provides the API as well as two data types, PFC_CNT and PFC_CFG, which are both 64-bit 2's complement integers.

Initialize the library

pfcInit()

Initializes the library. If the magic files exposed by pfc.ko (/sys/module/pfc/config and /sys/module/pfc/counts) cannot be opened, prints an error message.

Pin thread to single core

pfcPinThread(coreNum)

Pins the thread to one selected core. This is important both because core migration interferes with performance statistics, and because pfc.ko does not virtualize or track counter values in any way. It is thus crucial that the process occupies a single core and that no other processor occupy it.

Parse configurations

pfcParseCfg()

Takes a string describing a general-purpose counter event to track, and computes the 64-bit PFC_CFG value to be written to the MSR that would cause it to monitor said event.

The fixed-function performance counters are enabled using configuration 2 and disabled with configuration 0. No other configuration is allowed.

Read/Write Configs & Counts

    pfcRdCfgs(k, n, cfgs);
    pfcWrCfgs(k, n, cfgs);
    pfcRdCnts(k, n, cnts);
    pfcWrCnts(k, n, cnts);

Reads and writes hardware counter and configuration values for the n counters starting at counter k. On Haswell, counters 0, 1 and 2 are fixed-function counters, while counters 3, 4, 5 and 6 are general-purpose counters.

Timing Code

libpfc.h defines two assembler macros and one function for timing.

• PFCSTART(cnts) reads the 7 hardware counters using rdpmc in a carefully-timed manner and subtracts them out of the current value in cnt[0..6].

• PFCEND(cnts) reads the 7 hardware counters using rdpmc in a carefully-timed manner and adds them into cnt[0..6].

• Since PFCSTART/PFCEND have non-negligible but identical cost and highly-predictable behaviour, the bias that they introduce in the recorded counter values can be computed and removed.

  pfcRemoveBias(cnts, mul) measures the cost of a pair PFCSTART/PFCEND with nothing in-between with the current counter configurations, and subtracts mul copies of that cost out of cnts.

Therefore, to measure a snippet of code, one does as follows:

    /* Initialization */
    pfcInit();
    pfcPinThread(3);  /* Ideally, to any core other than 0 */
    
    /* Compute configurations, possibly using `pfcParseConfig()` */
    PFC_CFG cfgs[7] = {...};
    PFC_CNT cnts[7] = {0,0,0,0,0,0,0};
    
    /* Write configurations and starting values */
    pfcWrCfgs(0, 7, cfgs);
    pfcWrCnts(0, 7, cnts);
    
    /**** HOT SECTION ****/
    PFCSTART(cnts);
    /* Snippet to time */
    PFCEND(cnts);
    /**** END HOT SECTION ****/
    
    /* Bias compensation */
    pfcRemoveBias(cnts, 1);
    
    /* Use values in cnts[0..6]. */

An example of this process is in pfcdemo.c:71.