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 <dirent.h>
  12 #include <errno.h>
  13 #include <fcntl.h>
  14 #include <limits.h>
  15 #include <unistd.h>
  16 #include <pthread.h>
  17 #include <stdlib.h>
  18 #include <string.h>
  19 #include <sys/types.h>
  20
  21 #include <urcu/compiler.h>
  22
  23 #include "common/logging.h"
  24 #include "common/smp.h"
  25
  26 #define __max(a,b) ((a)>(b)?(a):(b))
  27
  28 static int num_possible_cpus_cache;
  29
  30 /*
  31  * As a fallback to parsing the CPU mask in "/sys/devices/system/cpu/possible",
  32  * iterate on all the folders in "/sys/devices/system/cpu" that start with
  33  * "cpu" followed by an integer, keep the highest CPU id encountered during
  34  * this iteration and add 1 to get a number of CPUs.
  35  *
  36  * Then get the value from sysconf(_SC_NPROCESSORS_CONF) as a fallback and
  37  * return the highest one.
  38  *
  39  * On Linux, using the value from sysconf can be unreliable since the way it
  40  * counts CPUs varies between C libraries and even between versions of the same
  41  * library. If we used it directly, getcpu() could return a value greater than
  42  * this sysconf, in which case the arrays indexed by processor would overflow.
  43  *
  44  * As another example, the MUSL libc implementation of the _SC_NPROCESSORS_CONF
  45  * sysconf does not return the number of configured CPUs in the system but
  46  * relies on the cpu affinity mask of the current task.
  47  *
  48  * Returns 0 or less on error.
  49  */
  50 int get_num_possible_cpus_fallback(void)
  51 {
  52         long max_cpuid = -1;
  53
  54         DIR *cpudir;
  55         struct dirent *entry;
  56
  57         cpudir = opendir("/sys/devices/system/cpu");
  58         if (cpudir == NULL)
  59                 goto end;
  60
  61         /*
  62          * Iterate on all directories named "cpu" followed by an integer.
  63          */
  64         while ((entry = readdir(cpudir))) {
  65                 if (entry->d_type == DT_DIR &&
  66                         strncmp(entry->d_name, "cpu", 3) == 0) {
  67
  68                         char *endptr;
  69                         long cpu_id;
  70
  71                         cpu_id = strtol(entry->d_name + 3, &endptr, 10);
  72                         if ((cpu_id < LONG_MAX) && (endptr != entry->d_name + 3)
  73                                         && (*endptr == '\0')) {
  74                                 if (cpu_id > max_cpuid)
  75                                         max_cpuid = cpu_id;
  76                         }
  77                 }
  78         }
  79
  80         /*
  81          * If the max CPU id is out of bound, set it to -1 so it results in a
  82          * CPU num of 0.
  83          */
  84         if (max_cpuid < 0 || max_cpuid > INT_MAX)
  85                 max_cpuid = -1;
  86
  87 end:
  88         /*
  89          * Get the sysconf value as a last resort. Keep the highest number.
  90          */
  91         return __max(sysconf(_SC_NPROCESSORS_CONF), max_cpuid + 1);
  92 }
  93
  94 /*
  95  * Get the CPU possible mask string from sysfs.
  96  *
  97  * buf: the buffer where the mask will be read.
  98  * max_bytes: the maximum number of bytes to write in the buffer.
  99  *
 100  * Returns the number of bytes read or -1 on error.
 101  */
 102 int get_possible_cpu_mask_from_sysfs(char *buf, size_t max_bytes)
 103 {
 104         ssize_t bytes_read = 0;
 105         size_t total_bytes_read = 0;
 106         int fd = -1, ret = -1;
 107
 108         if (buf == NULL)
 109                 goto end;
 110
 111         fd = open("/sys/devices/system/cpu/possible", O_RDONLY);
 112         if (fd < 0)
 113                 goto end;
 114
 115         do {
 116                 bytes_read = read(fd, buf + total_bytes_read,
 117                                 max_bytes - total_bytes_read);
 118
 119                 if (bytes_read < 0) {
 120                         if (errno == EINTR) {
 121                                 continue;       /* retry operation */
 122                         } else {
 123                                 goto end;
 124                         }
 125                 }
 126
 127                 total_bytes_read += bytes_read;
 128                 assert(total_bytes_read <= max_bytes);
 129         } while (max_bytes > total_bytes_read && bytes_read > 0);
 130
 131         /*
 132          * Make sure the mask read is a null terminated string.
 133          */
 134         if (total_bytes_read < max_bytes)
 135                 buf[total_bytes_read] = '\0';
 136         else
 137                 buf[max_bytes - 1] = '\0';
 138
 139         if (total_bytes_read > INT_MAX)
 140                 goto end;
 141         ret = (int) total_bytes_read;
 142 end:
 143         if (fd >= 0 && close(fd) < 0)
 144                 PERROR("close");
 145         return ret;
 146 }
 147
 148 /*
 149  * Get the number of CPUs from the possible cpu mask.
 150  *
 151  * pmask: the mask to parse.
 152  * len: the len of the mask excluding '\0'.
 153  *
 154  * Returns the number of possible CPUs from the mask or 0 on error.
 155  */
 156 int get_num_possible_cpus_from_mask(const char *pmask, size_t len)
 157 {
 158         ssize_t i;
 159         unsigned long cpu_index;
 160         char *endptr;
 161
 162         /* We need at least one char to read */
 163         if (len < 1)
 164                 goto error;
 165
 166         /* Start from the end to read the last CPU index. */
 167         for (i = len - 1; i > 0; i--) {
 168                 /* Break when we hit the first separator. */
 169                 if ((pmask[i] == ',') || (pmask[i] == '-')) {
 170                         i++;
 171                         break;
 172                 }
 173         }
 174
 175         cpu_index = strtoul(&pmask[i], &endptr, 10);
 176
 177         /*
 178          * If we read a CPU index, increment it by one to return a number of
 179          * CPUs.
 180          */
 181         if ((&pmask[i] != endptr) && (cpu_index < INT_MAX))
 182                 return (int) cpu_index + 1;
 183
 184 error:
 185         return 0;
 186 }
 187
 188 static void _get_num_possible_cpus(void)
 189 {
 190         int ret;
 191         char buf[LTTNG_UST_CPUMASK_SIZE];
 192
 193         /* Get the possible cpu mask from sysfs, fallback to sysconf. */
 194         ret = get_possible_cpu_mask_from_sysfs((char *) &buf, LTTNG_UST_CPUMASK_SIZE);
 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 }