--- name: "tracefs_instance_set_affinity" section: "3" platform: "linux" package: "libtracefs" version: "1.8.4-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/tracefs_instance_set_affinity" --- # tracefs\_instance\_set\_affinity(3) Sets or retrieves the affinity for an instance or top level for what CPUs enable tracing. ## NAME 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. ## SYNOPSIS ``` #include 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); ``` ## DESCRIPTION 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)](/CPU_SET/3.md). 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: ``` "100,000007fe" ``` 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)](/free/3.md). The **tracefs\_instance\_get\_affinity\_set()** will set all the bits in the passed in cpu set (from [**CPU\_SET**(3)](/CPU_SET/3.md)). 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)](/free/3.md). ## RETURN VALUE 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)](/free/3.md), or NULL on error. The function **tracefs\_instance\_get\_affinity\_set()** returns the number of CPUs that were found set, or -1 on error. ## ERRORS 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. ## EXAMPLE ``` #include #include #include #include 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); free(c); if (argc < 2) { tracefs_instance_set_affinity(NULL, NULL); exit(0); } /* Show example using a set */ if (argc == 2 && !strchr(argv[1],',')) { cpu1 = atoi(argv[1]); c = strchr(argv[1], '-'); if (c++) cpu2 = atoi(c); else cpu2 = cpu1; if (cpu2 < cpu1) { fprintf(stderr, "Invalid CPU range\n"); exit(-1); } 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); CPU_FREE(set); exit(0); } trace_seq_init(&seq); for (i = 1; i < argc; i++) { if (i > 1) trace_seq_putc(&seq, ','); trace_seq_puts(&seq, argv[i]); } trace_seq_terminate(&seq); tracefs_instance_set_affinity(NULL, seq.buffer); trace_seq_destroy(&seq); exit(0); return 0; } ``` ## FILES ``` tracefs.h 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. ``` ## SEE ALSO [**libtracefs**(3)](/libtracefs/3.md), [**libtraceevent**(3)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md) ## AUTHOR ``` Steven Rostedt Tzvetomir Stoyanov ``` ## REPORTING BUGS Report bugs to \<**linux-trace-devel@vger.kernel.org**\[3\]\> ## LICENSE libtracefs is Free Software licensed under the GNU LGPL 2.1 ## RESOURCES **https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/** ## COPYING Copyright (C) 2020 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL). ## NOTES - 1\. rostedt@goodmis.org mailto:rostedt@goodmis.org - 2\. tz.stoyanov@gmail.com mailto:tz.stoyanov@gmail.com - 3\. linux-trace-devel@vger.kernel.org mailto:linux-trace-devel@vger.kernel.org