doc/man: only mention `-llttng-ust-common` in synopses (conditionally)

[lttng-ust.git] / src / common / getcpu.h

/*
 * SPDX-License-Identifier: LGPL-2.1-only
 *
 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 */

#ifndef _UST_COMMON_GETCPU_H
#define _UST_COMMON_GETCPU_H

#include <urcu/compiler.h>
#include <urcu/system.h>
#include <urcu/arch.h>

#include <lttng/ust-getcpu.h>

/*
 * Function pointer to the user provided getcpu callback, can be set at library
 * initialization by a dlopened plugin or at runtime by a user by calling
 * lttng_ust_getcpu_override() from the public API.
 *
 * This is an ABI symbol of liblttng-ust-common accessed by other libraries
 * through the static inline function in this file. It is initialised in the
 * liblttng-ust-common constructor.
 */
extern int (*lttng_ust_get_cpu_sym)(void);

#ifdef LTTNG_UST_DEBUG_VALGRIND

/*
 * Fallback on cpu 0 if liblttng-ust is build with Valgrind support.
 * get_cpu() returns the current CPU number. It may change due to
 * migration, so it is only statistically accurate.
 */
static inline
int lttng_ust_get_cpu_internal(void)
{
	return 0;
}

#else

/*
 * sched_getcpu.
 */
#ifdef __linux__

#if !HAVE_SCHED_GETCPU
#include <sys/syscall.h>
#define __getcpu(cpu, node, cache)	syscall(__NR_getcpu, cpu, node, cache)
/*
 * If getcpu is not implemented in the kernel, use cpu 0 as fallback.
 */
static inline
int lttng_ust_get_cpu_internal(void)
{
	int cpu, ret;

	ret = __getcpu(&cpu, NULL, NULL);
	if (caa_unlikely(ret < 0))
		return 0;
	return cpu;
}
#else /* HAVE_SCHED_GETCPU */
#include <sched.h>

/*
 * If getcpu is not implemented in the kernel, use cpu 0 as fallback.
 */
static inline
int lttng_ust_get_cpu_internal(void)
{
	int cpu;

	cpu = sched_getcpu();
	if (caa_unlikely(cpu < 0))
		return 0;
	return cpu;
}
#endif	/* HAVE_SCHED_GETCPU */

#elif (defined(__FreeBSD__) || defined(__CYGWIN__))

/*
 * FreeBSD and Cygwin do not allow query of CPU ID. Always use CPU
 * number 0, with the assocated performance degradation on SMP.
 */
static inline
int lttng_ust_get_cpu_internal(void)
{
	return 0;
}

#else
#error "Please add support for your OS into liblttng-ust/compat.h."
#endif

#endif

static inline
int lttng_ust_get_cpu(void)
{
	int (*lttng_ust_get_cpu_current)(void) = CMM_LOAD_SHARED(lttng_ust_get_cpu_sym);

	/*
	 * Fallback to the internal getcpu implementation if no override was
	 * provided the user.
	 */
	if (caa_likely(!lttng_ust_get_cpu_current)) {
		return lttng_ust_get_cpu_internal();
	} else {
		return lttng_ust_get_cpu_current();
	}
}

#endif /* _LTTNG_GETCPU_H */