fix: num_possible_cpus() with hot-unplugged CPUs

[lttng-ust.git] / libringbuffer / smp.h

diff --git a/libringbuffer/smp.h b/libringbuffer/smp.h
index 479a9dc60a01c9810d166ed1944e8fc7dfb4ecb0..1a880994bbe67a656c1e0ff499de67939df0390a 100644 (file)
--- a/libringbuffer/smp.h
+++ b/libringbuffer/smp.h
@@ -30,13 +30,53 @@
 #define PER_CPU_MEM_SIZE       4096
 
 extern int __num_possible_cpus;
+
+/*
+ * Get the CPU possible mask string from sysfs.
+ *
+ * buf: the buffer where the mask will be read.
+ * max_bytes: the maximum number of bytes to write in the buffer.
+ *
+ * Returns the number of bytes read or -1 on error.
+ */
+int get_possible_cpu_mask_from_sysfs(char *buf, size_t max_bytes)
+       __attribute__((visibility("hidden")));
+
+/*
+ * Get the number of possible CPUs in the system from either
+ * sysconf(_SC_NPROCESSORS_CONF) or some other mechanism depending on the libc.
+ *
+ * Returns the number of possible CPUs in the system or 0 on error.
+ */
+int get_num_possible_cpus_fallback(void)
+       __attribute__((visibility("hidden")));
+
+/*
+ * Get the number of CPUs from the possible cpu mask.
+ *
+ * pmask: the mask to parse.
+ * len: the len of the mask excluding '\0'.
+ *
+ * Returns the number of possible CPUs from the mask or 0 on error.
+ */
+int get_num_possible_cpus_from_mask(const char *pmask, size_t len)
+       __attribute__((visibility("hidden")));
+
 extern void _get_num_possible_cpus(void);
 
+/*
+ * Returns the total number of CPUs in the system. If the cache is not yet
+ * initialized, get the value from "/sys/devices/system/cpu/possible" or
+ * fallback to sysconf and cache it.
+ *
+ * If all methods fail, don't populate the cache and return 0.
+ */
 static inline
 int num_possible_cpus(void)
 {
-       if (!__num_possible_cpus)
+       if (caa_unlikely(!__num_possible_cpus))
                _get_num_possible_cpus();
+
        return __num_possible_cpus;
 }