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LIBTRACEFS(3) libtracefs Manual LIBTRACEFS(3)


tracefs_instance_set_affinity, tracefs_instance_set_affinity_set, tracefs_instance_set_affinity_raw, tracefs_instance_get_affinity, tracefs_instance_get_affinity_set, tracefs_instance_get_affinity_raw - Sets or retrieves the affinity for an instance or top level for what CPUs enable tracing.


#include <tracefs.h>
int tracefs_instance_set_affinity(struct tracefs_instance *instance, const char *cpu_str);
int tracefs_instance_set_affinity_set(struct tracefs_instance *instance, cpu_set_t *set, size_t set_size);
int tracefs_instance_set_affinity_raw(struct tracefs_instance *instance, const char *mask);
char *tracefs_instance_get_affinity(struct tracefs_instance *instance);
int tracefs_instance_get_affinity_set(struct tracefs_instance *instance, cpu_set_t *set, size_t set_size);
char *tracefs_instance_get_affinity_raw(struct tracefs_instance *instance);


These functions set or retrieve the CPU affinity that limits what CPUs will have tracing enabled for a given instance defined by the instance parameter. If instance is NULL, then the top level instance is affected.

The tracefs_instance_set_affinity() function takes a string cpu_str that is a list of CPUs to set the affinity for. If cpu_str is NULL, then all the CPUs in the system will be set. The format of cpu_str is a comma delimited string of decimal numbers with no spaces. A range may be specified by a hyphen.

For example: "1,4,6-8"

The numbers do not need to be in order except for ranges, where the second number must be equal to or greater than the first.

The tracefs_instance_set_affinity_set() function takes a CPU set defined by CPU_SET(3). The size of the set defined by set_size is the size in bytes of set. If set is NULL then all the CPUs on the system will be set, and set_size is ignored.

The tracefs_instance_set_affinity_raw() function takes a string that holds a hexidecimal bitmask, where each 32 bits is separated by a comma. For a machine with more that 32 CPUs, to set CPUS 1-10 and CPU 40:


Where the above is a hex representation of bits 1-10 and bit 40 being set.

The tracefs_instance_get_affinity() will retrieve the affinity in a human readable form.

For example: "1,4,6-8"

The string returned must be freed with free(3).

The tracefs_instance_get_affinity_set() will set all the bits in the passed in cpu set (from CPU_SET(3)). Note it will not clear any bits that are already set in the set but the CPUs are not. If only the bits for the CPUs that are enabled should be set, a CPU_ZERO_S() should be performed on the set before calling this function.

The tracefs_instance_get_affinity_raw() will simply read the instance tracing_cpumask and return that string. The returned string must be freed with free(3).


All the set functions return 0 on success and -1 on error.

The functions tracefs_instance_get_affinity() and tracefs_instance_get_affinity_raw() returns an allocated string that must be freed with free(3), or NULL on error.

The function tracefs_instance_get_affinity_set() returns the number of CPUs that were found set, or -1 on error.


The following errors are for all the above calls:

EFBIG if a CPU is set that is greater than what is in the system.

EINVAL One of the parameters was invalid.

The following errors are for tracefs_instance_set_affinity() and tracefs_instance_set_affinity_set():

ENOMEM Memory allocation error.

ENODEV dynamic events of requested type are not configured for the running kernel.

The following errors are just for tracefs_instance_set_affinity()

EACCES The cpu_str was modified by another thread when processing it.


#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <tracefs.h>
int main (int argc, char **argv)

struct trace_seq seq;
cpu_set_t *set;
size_t set_size;
char *c;
int cpu1;
int cpu2;
int i;
c = tracefs_instance_get_affinity(NULL);
printf("The affinity was %s\n", c);
if (argc < 2) {
tracefs_instance_set_affinity(NULL, NULL);
/* Show example using a set */
if (argc == 2 && !strchr(argv[1],',')) {
cpu1 = atoi(argv[1]);
c = strchr(argv[1], '-');
if (c++)
cpu2 = atoi(c);
cpu2 = cpu1;
if (cpu2 < cpu1) {
fprintf(stderr, "Invalid CPU range\n");
set = CPU_ALLOC(cpu2 + 1);
set_size = CPU_ALLOC_SIZE(cpu2 + 1);
CPU_ZERO_S(set_size, set);
for ( ; cpu1 <= cpu2; cpu1++)
CPU_SET(cpu1, set);
tracefs_instance_set_affinity_set(NULL, set, set_size);
for (i = 1; i < argc; i++) {
if (i > 1)
trace_seq_putc(&seq, ',');
trace_seq_puts(&seq, argv[i]);
tracefs_instance_set_affinity(NULL, seq.buffer);
return 0; }



Header file to include in order to have access to the library APIs. -ltracefs
Linker switch to add when building a program that uses the library.


libtracefs(3), libtraceevent(3), trace-cmd(1)


Steven Rostedt <[1]>
Tzvetomir Stoyanov <[2]>


Report bugs to <[3]>


libtracefs is Free Software licensed under the GNU LGPL 2.1



Copyright (C) 2020 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).


06/15/2023 libtracefs