lib/percpu_counter.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Fast batching percpu counters.
   4  */
   5
   6 #include <linux/percpu_counter.h>
   7 #include <linux/mutex.h>
   8 #include <linux/init.h>
   9 #include <linux/cpu.h>
  10 #include <linux/module.h>
  11 #include <linux/debugobjects.h>
  12
  13 #ifdef CONFIG_HOTPLUG_CPU
  14 static LIST_HEAD(percpu_counters);
  15 static DEFINE_SPINLOCK(percpu_counters_lock);
  16 #endif
  17
  18 #ifdef CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER
  19
  20 static const struct debug_obj_descr percpu_counter_debug_descr;
  21
  22 static bool percpu_counter_fixup_free(void *addr, enum debug_obj_state state)
  23 {
  24         struct percpu_counter *fbc = addr;
  25
  26         switch (state) {
  27         case ODEBUG_STATE_ACTIVE:
  28                 percpu_counter_destroy(fbc);
  29                 debug_object_free(fbc, &percpu_counter_debug_descr);
  30                 return true;
  31         default:
  32                 return false;
  33         }
  34 }
  35
  36 static const struct debug_obj_descr percpu_counter_debug_descr = {
  37         .name           = "percpu_counter",
  38         .fixup_free     = percpu_counter_fixup_free,
  39 };
  40
  41 static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
  42 {
  43         debug_object_init(fbc, &percpu_counter_debug_descr);
  44         debug_object_activate(fbc, &percpu_counter_debug_descr);
  45 }
  46
  47 static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
  48 {
  49         debug_object_deactivate(fbc, &percpu_counter_debug_descr);
  50         debug_object_free(fbc, &percpu_counter_debug_descr);
  51 }
  52
  53 #else   /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */
  54 static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
  55 { }
  56 static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
  57 { }
  58 #endif  /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */
  59
  60 void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
  61 {
  62         int cpu;
  63         unsigned long flags;
  64
  65         raw_spin_lock_irqsave(&fbc->lock, flags);
  66         for_each_possible_cpu(cpu) {
  67                 s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
  68                 *pcount = 0;
  69         }
  70         fbc->count = amount;
  71         raw_spin_unlock_irqrestore(&fbc->lock, flags);
  72 }
  73 EXPORT_SYMBOL(percpu_counter_set);
  74
  75 /*
  76  * Add to a counter while respecting batch size.
  77  *
  78  * There are 2 implementations, both dealing with the following problem:
  79  *
  80  * The decision slow path/fast path and the actual update must be atomic.
  81  * Otherwise a call in process context could check the current values and
  82  * decide that the fast path can be used. If now an interrupt occurs before
  83  * the this_cpu_add(), and the interrupt updates this_cpu(*fbc->counters),
  84  * then the this_cpu_add() that is executed after the interrupt has completed
  85  * can produce values larger than "batch" or even overflows.
  86  */
  87 #ifdef CONFIG_HAVE_CMPXCHG_LOCAL
  88 /*
  89  * Safety against interrupts is achieved in 2 ways:
  90  * 1. the fast path uses local cmpxchg (note: no lock prefix)
  91  * 2. the slow path operates with interrupts disabled
  92  */
  93 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
  94 {
  95         s64 count;
  96         unsigned long flags;
  97
  98         count = this_cpu_read(*fbc->counters);
  99         do {
 100                 if (unlikely(abs(count + amount) >= batch)) {
 101                         raw_spin_lock_irqsave(&fbc->lock, flags);
 102                         /*
 103                          * Note: by now we might have migrated to another CPU
 104                          * or the value might have changed.
 105                          */
 106                         count = __this_cpu_read(*fbc->counters);
 107                         fbc->count += count + amount;
 108                         __this_cpu_sub(*fbc->counters, count);
 109                         raw_spin_unlock_irqrestore(&fbc->lock, flags);
 110                         return;
 111                 }
 112         } while (!this_cpu_try_cmpxchg(*fbc->counters, &count, count + amount));
 113 }
 114 #else
 115 /*
 116  * local_irq_save() is used to make the function irq safe:
 117  * - The slow path would be ok as protected by an irq-safe spinlock.
 118  * - this_cpu_add would be ok as it is irq-safe by definition.
 119  */
 120 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
 121 {
 122         s64 count;
 123         unsigned long flags;
 124
 125         local_irq_save(flags);
 126         count = __this_cpu_read(*fbc->counters) + amount;
 127         if (abs(count) >= batch) {
 128                 raw_spin_lock(&fbc->lock);
 129                 fbc->count += count;
 130                 __this_cpu_sub(*fbc->counters, count - amount);
 131                 raw_spin_unlock(&fbc->lock);
 132         } else {
 133                 this_cpu_add(*fbc->counters, amount);
 134         }
 135         local_irq_restore(flags);
 136 }
 137 #endif
 138 EXPORT_SYMBOL(percpu_counter_add_batch);
 139
 140 /*
 141  * For percpu_counter with a big batch, the devication of its count could
 142  * be big, and there is requirement to reduce the deviation, like when the
 143  * counter's batch could be runtime decreased to get a better accuracy,
 144  * which can be achieved by running this sync function on each CPU.
 145  */
 146 void percpu_counter_sync(struct percpu_counter *fbc)
 147 {
 148         unsigned long flags;
 149         s64 count;
 150
 151         raw_spin_lock_irqsave(&fbc->lock, flags);
 152         count = __this_cpu_read(*fbc->counters);
 153         fbc->count += count;
 154         __this_cpu_sub(*fbc->counters, count);
 155         raw_spin_unlock_irqrestore(&fbc->lock, flags);
 156 }
 157 EXPORT_SYMBOL(percpu_counter_sync);
 158
 159 /*
 160  * Add up all the per-cpu counts, return the result.  This is a more accurate
 161  * but much slower version of percpu_counter_read_positive().
 162  *
 163  * We use the cpu mask of (cpu_online_mask | cpu_dying_mask) to capture sums
 164  * from CPUs that are in the process of being taken offline. Dying cpus have
 165  * been removed from the online mask, but may not have had the hotplug dead
 166  * notifier called to fold the percpu count back into the global counter sum.
 167  * By including dying CPUs in the iteration mask, we avoid this race condition
 168  * so __percpu_counter_sum() just does the right thing when CPUs are being taken
 169  * offline.
 170  */
 171 s64 __percpu_counter_sum(struct percpu_counter *fbc)
 172 {
 173         s64 ret;
 174         int cpu;
 175         unsigned long flags;
 176
 177         raw_spin_lock_irqsave(&fbc->lock, flags);
 178         ret = fbc->count;
 179         for_each_cpu_or(cpu, cpu_online_mask, cpu_dying_mask) {
 180                 s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
 181                 ret += *pcount;
 182         }
 183         raw_spin_unlock_irqrestore(&fbc->lock, flags);
 184         return ret;
 185 }
 186 EXPORT_SYMBOL(__percpu_counter_sum);
 187
 188 int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 189                                gfp_t gfp, u32 nr_counters,
 190                                struct lock_class_key *key)
 191 {
 192         unsigned long flags __maybe_unused;
 193         size_t counter_size;
 194         s32 __percpu *counters;
 195         u32 i;
 196
 197         counter_size = ALIGN(sizeof(*counters), __alignof__(*counters));
 198         counters = __alloc_percpu_gfp(nr_counters * counter_size,
 199                                       __alignof__(*counters), gfp);
 200         if (!counters) {
 201                 fbc[0].counters = NULL;
 202                 return -ENOMEM;
 203         }
 204
 205         for (i = 0; i < nr_counters; i++) {
 206                 raw_spin_lock_init(&fbc[i].lock);
 207                 lockdep_set_class(&fbc[i].lock, key);
 208 #ifdef CONFIG_HOTPLUG_CPU
 209                 INIT_LIST_HEAD(&fbc[i].list);
 210 #endif
 211                 fbc[i].count = amount;
 212                 fbc[i].counters = (void __percpu *)counters + i * counter_size;
 213
 214                 debug_percpu_counter_activate(&fbc[i]);
 215         }
 216
 217 #ifdef CONFIG_HOTPLUG_CPU
 218         spin_lock_irqsave(&percpu_counters_lock, flags);
 219         for (i = 0; i < nr_counters; i++)
 220                 list_add(&fbc[i].list, &percpu_counters);
 221         spin_unlock_irqrestore(&percpu_counters_lock, flags);
 222 #endif
 223         return 0;
 224 }
 225 EXPORT_SYMBOL(__percpu_counter_init_many);
 226
 227 void percpu_counter_destroy_many(struct percpu_counter *fbc, u32 nr_counters)
 228 {
 229         unsigned long flags __maybe_unused;
 230         u32 i;
 231
 232         if (WARN_ON_ONCE(!fbc))
 233                 return;
 234
 235         if (!fbc[0].counters)
 236                 return;
 237
 238         for (i = 0; i < nr_counters; i++)
 239                 debug_percpu_counter_deactivate(&fbc[i]);
 240
 241 #ifdef CONFIG_HOTPLUG_CPU
 242         spin_lock_irqsave(&percpu_counters_lock, flags);
 243         for (i = 0; i < nr_counters; i++)
 244                 list_del(&fbc[i].list);
 245         spin_unlock_irqrestore(&percpu_counters_lock, flags);
 246 #endif
 247
 248         free_percpu(fbc[0].counters);
 249
 250         for (i = 0; i < nr_counters; i++)
 251                 fbc[i].counters = NULL;
 252 }
 253 EXPORT_SYMBOL(percpu_counter_destroy_many);
 254
 255 int percpu_counter_batch __read_mostly = 32;
 256 EXPORT_SYMBOL(percpu_counter_batch);
 257
 258 static int compute_batch_value(unsigned int cpu)
 259 {
 260         int nr = num_online_cpus();
 261
 262         percpu_counter_batch = max(32, nr*2);
 263         return 0;
 264 }
 265
 266 static int percpu_counter_cpu_dead(unsigned int cpu)
 267 {
 268 #ifdef CONFIG_HOTPLUG_CPU
 269         struct percpu_counter *fbc;
 270
 271         compute_batch_value(cpu);
 272
 273         spin_lock_irq(&percpu_counters_lock);
 274         list_for_each_entry(fbc, &percpu_counters, list) {
 275                 s32 *pcount;
 276
 277                 raw_spin_lock(&fbc->lock);
 278                 pcount = per_cpu_ptr(fbc->counters, cpu);
 279                 fbc->count += *pcount;
 280                 *pcount = 0;
 281                 raw_spin_unlock(&fbc->lock);
 282         }
 283         spin_unlock_irq(&percpu_counters_lock);
 284 #endif
 285         return 0;
 286 }
 287
 288 /*
 289  * Compare counter against given value.
 290  * Return 1 if greater, 0 if equal and -1 if less
 291  */
 292 int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)
 293 {
 294         s64     count;
 295
 296         count = percpu_counter_read(fbc);
 297         /* Check to see if rough count will be sufficient for comparison */
 298         if (abs(count - rhs) > (batch * num_online_cpus())) {
 299                 if (count > rhs)
 300                         return 1;
 301                 else
 302                         return -1;
 303         }
 304         /* Need to use precise count */
 305         count = percpu_counter_sum(fbc);
 306         if (count > rhs)
 307                 return 1;
 308         else if (count < rhs)
 309                 return -1;
 310         else
 311                 return 0;
 312 }
 313 EXPORT_SYMBOL(__percpu_counter_compare);
 314
 315 /*
 316  * Compare counter, and add amount if total is: less than or equal to limit if
 317  * amount is positive, or greater than or equal to limit if amount is negative.
 318  * Return true if amount is added, or false if total would be beyond the limit.
 319  *
 320  * Negative limit is allowed, but unusual.
 321  * When negative amounts (subs) are given to percpu_counter_limited_add(),
 322  * the limit would most naturally be 0 - but other limits are also allowed.
 323  *
 324  * Overflow beyond S64_MAX is not allowed for: counter, limit and amount
 325  * are all assumed to be sane (far from S64_MIN and S64_MAX).
 326  */
 327 bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 328                                   s64 limit, s64 amount, s32 batch)
 329 {
 330         s64 count;
 331         s64 unknown;
 332         unsigned long flags;
 333         bool good = false;
 334
 335         if (amount == 0)
 336                 return true;
 337
 338         local_irq_save(flags);
 339         unknown = batch * num_online_cpus();
 340         count = __this_cpu_read(*fbc->counters);
 341
 342         /* Skip taking the lock when safe */
 343         if (abs(count + amount) <= batch &&
 344             ((amount > 0 && fbc->count + unknown <= limit) ||
 345              (amount < 0 && fbc->count - unknown >= limit))) {
 346                 this_cpu_add(*fbc->counters, amount);
 347                 local_irq_restore(flags);
 348                 return true;
 349         }
 350
 351         raw_spin_lock(&fbc->lock);
 352         count = fbc->count + amount;
 353
 354         /* Skip percpu_counter_sum() when safe */
 355         if (amount > 0) {
 356                 if (count - unknown > limit)
 357                         goto out;
 358                 if (count + unknown <= limit)
 359                         good = true;
 360         } else {
 361                 if (count + unknown < limit)
 362                         goto out;
 363                 if (count - unknown >= limit)
 364                         good = true;
 365         }
 366
 367         if (!good) {
 368                 s32 *pcount;
 369                 int cpu;
 370
 371                 for_each_cpu_or(cpu, cpu_online_mask, cpu_dying_mask) {
 372                         pcount = per_cpu_ptr(fbc->counters, cpu);
 373                         count += *pcount;
 374                 }
 375                 if (amount > 0) {
 376                         if (count > limit)
 377                                 goto out;
 378                 } else {
 379                         if (count < limit)
 380                                 goto out;
 381                 }
 382                 good = true;
 383         }
 384
 385         count = __this_cpu_read(*fbc->counters);
 386         fbc->count += count + amount;
 387         __this_cpu_sub(*fbc->counters, count);
 388 out:
 389         raw_spin_unlock(&fbc->lock);
 390         local_irq_restore(flags);
 391         return good;
 392 }
 393
 394 static int __init percpu_counter_startup(void)
 395 {
 396         int ret;
 397
 398         ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "lib/percpu_cnt:online",
 399                                 compute_batch_value, NULL);
 400         WARN_ON(ret < 0);
 401         ret = cpuhp_setup_state_nocalls(CPUHP_PERCPU_CNT_DEAD,
 402                                         "lib/percpu_cnt:dead", NULL,
 403                                         percpu_counter_cpu_dead);
 404         WARN_ON(ret < 0);
 405         return 0;
 406 }
 407 module_init(percpu_counter_startup);