src/common/smp.c

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-only
   3  *
   4  * Copyright (C) 2011-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  * Copyright (C) 2019 Michael Jeanson <mjeanson@efficios.com>
   6  */
   7
   8 #define _LGPL_SOURCE
   9 #include <assert.h>
  10 #include <ctype.h>
  11 #include <errno.h>
  12 #include <fcntl.h>
  13 #include <unistd.h>
  14 #include <pthread.h>
  15 #include <stdlib.h>
  16
  17 #include <urcu/compiler.h>
  18
  19 #include "common/align.h"
  20 #include "common/logging.h"
  21 #include "common/smp.h"
  22
  23 static int num_possible_cpus_cache;
  24
  25 #if (defined(__GLIBC__) || defined( __UCLIBC__))
  26 int get_num_possible_cpus_fallback(void)
  27 {
  28         /* On Linux, when some processors are offline
  29          * _SC_NPROCESSORS_CONF counts the offline
  30          * processors, whereas _SC_NPROCESSORS_ONLN
  31          * does not. If we used _SC_NPROCESSORS_ONLN,
  32          * getcpu() could return a value greater than
  33          * this sysconf, in which case the arrays
  34          * indexed by processor would overflow.
  35          */
  36         return sysconf(_SC_NPROCESSORS_CONF);
  37 }
  38
  39 #else
  40
  41 /*
  42  * The MUSL libc implementation of the _SC_NPROCESSORS_CONF sysconf does not
  43  * return the number of configured CPUs in the system but relies on the cpu
  44  * affinity mask of the current task.
  45  *
  46  * So instead we use a strategy similar to GLIBC's, counting the cpu
  47  * directories in "/sys/devices/system/cpu" and fallback on the value from
  48  * sysconf if it fails.
  49  */
  50
  51 #include <dirent.h>
  52 #include <limits.h>
  53 #include <stdlib.h>
  54 #include <string.h>
  55 #include <sys/types.h>
  56
  57 #define __max(a,b) ((a)>(b)?(a):(b))
  58
  59 int get_num_possible_cpus_fallback(void)
  60 {
  61         int count = 0;
  62         DIR *cpudir;
  63         struct dirent *entry;
  64
  65         cpudir = opendir("/sys/devices/system/cpu");
  66         if (cpudir == NULL)
  67                 goto end;
  68
  69         /*
  70          * Count the number of directories named "cpu" followed by and
  71          * integer. This is the same strategy as glibc uses.
  72          */
  73         while ((entry = readdir(cpudir))) {
  74                 if (entry->d_type == DT_DIR &&
  75                         strncmp(entry->d_name, "cpu", 3) == 0) {
  76
  77                         char *endptr;
  78                         unsigned long cpu_num;
  79
  80                         cpu_num = strtoul(entry->d_name + 3, &endptr, 10);
  81                         if ((cpu_num < ULONG_MAX) && (endptr != entry->d_name + 3)
  82                                         && (*endptr == '\0')) {
  83                                 count++;
  84                         }
  85                 }
  86         }
  87
  88 end:
  89         /*
  90          * Get the sysconf value as a fallback. Keep the highest number.
  91          */
  92         return __max(sysconf(_SC_NPROCESSORS_CONF), count);
  93 }
  94 #endif
  95
  96 /*
  97  * Get the CPU possible mask string from sysfs.
  98  *
  99  * buf: the buffer where the mask will be read.
 100  * max_bytes: the maximum number of bytes to write in the buffer.
 101  *
 102  * Returns the number of bytes read or -1 on error.
 103  */
 104 int get_possible_cpu_mask_from_sysfs(char *buf, size_t max_bytes)
 105 {
 106         ssize_t bytes_read = 0;
 107         size_t total_bytes_read = 0;
 108         int fd = 0;
 109
 110         if (buf == NULL)
 111                 return -1;
 112
 113         fd = open("/sys/devices/system/cpu/possible", O_RDONLY);
 114         if (fd < 0)
 115                 return -1;
 116
 117         do {
 118                 bytes_read = read(fd, buf + total_bytes_read,
 119                                 max_bytes - total_bytes_read);
 120
 121                 if (bytes_read < 0) {
 122                         if (errno == EINTR) {
 123                                 continue;       /* retry operation */
 124                         } else {
 125                                 return -1;
 126                         }
 127                 }
 128
 129                 total_bytes_read += bytes_read;
 130                 assert(total_bytes_read <= max_bytes);
 131         } while (max_bytes > total_bytes_read && bytes_read > 0);
 132
 133         if (close(fd))
 134                 PERROR("close");
 135
 136         /*
 137          * Make sure the mask read is a null terminated string.
 138          */
 139         if (total_bytes_read < max_bytes)
 140                 buf[total_bytes_read] = '\0';
 141         else
 142                 buf[max_bytes - 1] = '\0';
 143
 144         return total_bytes_read;
 145 }
 146
 147 /*
 148  * Get the number of CPUs from the possible cpu mask.
 149  *
 150  * pmask: the mask to parse.
 151  * len: the len of the mask excluding '\0'.
 152  *
 153  * Returns the number of possible CPUs from the mask or 0 on error.
 154  */
 155 int get_num_possible_cpus_from_mask(const char *pmask, size_t len)
 156 {
 157         ssize_t i;
 158         unsigned long cpu_index;
 159         char *endptr;
 160
 161         /* We need at least one char to read */
 162         if (len < 1)
 163                 goto error;
 164
 165         /* Start from the end to read the last CPU index. */
 166         for (i = len - 1; i > 0; i--) {
 167                 /* Break when we hit the first separator. */
 168                 if ((pmask[i] == ',') || (pmask[i] == '-')) {
 169                         i++;
 170                         break;
 171                 }
 172         }
 173
 174         cpu_index = strtoul(&pmask[i], &endptr, 10);
 175
 176         /*
 177          * If we read a CPU index, increment it by one to return a number of
 178          * CPUs.
 179          */
 180         if ((&pmask[i] != endptr) && (cpu_index < INT_MAX))
 181                 return (int) cpu_index + 1;
 182
 183 error:
 184         return 0;
 185 }
 186
 187 static void _get_num_possible_cpus(void)
 188 {
 189         int ret;
 190         int buf_len = LTTNG_UST_PAGE_SIZE;
 191         char buf[buf_len];
 192
 193         /* Get the possible cpu mask from sysfs, fallback to sysconf. */
 194         ret = get_possible_cpu_mask_from_sysfs((char *) &buf, buf_len);
 195         if (ret <= 0)
 196                 goto fallback;
 197
 198         /* Parse the possible cpu mask, on failure fallback to sysconf. */
 199         ret = get_num_possible_cpus_from_mask((char *) &buf, ret);
 200         if (ret > 0)
 201                 goto end;
 202
 203 fallback:
 204         /* Fallback to sysconf. */
 205         ret = get_num_possible_cpus_fallback();
 206
 207 end:
 208         /* If all methods failed, don't store the value. */
 209         if (ret < 1)
 210                 return;
 211
 212         num_possible_cpus_cache = ret;
 213 }
 214
 215 /*
 216  * Returns the total number of CPUs in the system. If the cache is not yet
 217  * initialized, get the value from "/sys/devices/system/cpu/possible" or
 218  * fallback to sysconf and cache it.
 219  *
 220  * If all methods fail, don't populate the cache and return 0.
 221  */
 222 int num_possible_cpus(void)
 223 {
 224         if (caa_unlikely(!num_possible_cpus_cache))
 225                 _get_num_possible_cpus();
 226
 227         return num_possible_cpus_cache;
 228 }