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  * local_irq_save() is needed to make the function irq safe:
  77  * - The slow path would be ok as protected by an irq-safe spinlock.
  78  * - this_cpu_add would be ok as it is irq-safe by definition.
  79  * But:
  80  * The decision slow path/fast path and the actual update must be atomic, too.
  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 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
  88 {
  89         s64 count;
  90         unsigned long flags;
  91
  92         local_irq_save(flags);
  93         count = __this_cpu_read(*fbc->counters) + amount;
  94         if (abs(count) >= batch) {
  95                 raw_spin_lock(&fbc->lock);
  96                 fbc->count += count;
  97                 __this_cpu_sub(*fbc->counters, count - amount);
  98                 raw_spin_unlock(&fbc->lock);
  99         } else {
 100                 this_cpu_add(*fbc->counters, amount);
 101         }
 102         local_irq_restore(flags);
 103 }
 104 EXPORT_SYMBOL(percpu_counter_add_batch);
 105
 106 /*
 107  * For percpu_counter with a big batch, the devication of its count could
 108  * be big, and there is requirement to reduce the deviation, like when the
 109  * counter's batch could be runtime decreased to get a better accuracy,
 110  * which can be achieved by running this sync function on each CPU.
 111  */
 112 void percpu_counter_sync(struct percpu_counter *fbc)
 113 {
 114         unsigned long flags;
 115         s64 count;
 116
 117         raw_spin_lock_irqsave(&fbc->lock, flags);
 118         count = __this_cpu_read(*fbc->counters);
 119         fbc->count += count;
 120         __this_cpu_sub(*fbc->counters, count);
 121         raw_spin_unlock_irqrestore(&fbc->lock, flags);
 122 }
 123 EXPORT_SYMBOL(percpu_counter_sync);
 124
 125 /*
 126  * Add up all the per-cpu counts, return the result.  This is a more accurate
 127  * but much slower version of percpu_counter_read_positive().
 128  *
 129  * We use the cpu mask of (cpu_online_mask | cpu_dying_mask) to capture sums
 130  * from CPUs that are in the process of being taken offline. Dying cpus have
 131  * been removed from the online mask, but may not have had the hotplug dead
 132  * notifier called to fold the percpu count back into the global counter sum.
 133  * By including dying CPUs in the iteration mask, we avoid this race condition
 134  * so __percpu_counter_sum() just does the right thing when CPUs are being taken
 135  * offline.
 136  */
 137 s64 __percpu_counter_sum(struct percpu_counter *fbc)
 138 {
 139         s64 ret;
 140         int cpu;
 141         unsigned long flags;
 142
 143         raw_spin_lock_irqsave(&fbc->lock, flags);
 144         ret = fbc->count;
 145         for_each_cpu_or(cpu, cpu_online_mask, cpu_dying_mask) {
 146                 s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
 147                 ret += *pcount;
 148         }
 149         raw_spin_unlock_irqrestore(&fbc->lock, flags);
 150         return ret;
 151 }
 152 EXPORT_SYMBOL(__percpu_counter_sum);
 153
 154 int __percpu_counter_init(struct percpu_counter *fbc, s64 amount, gfp_t gfp,
 155                           struct lock_class_key *key)
 156 {
 157         unsigned long flags __maybe_unused;
 158
 159         raw_spin_lock_init(&fbc->lock);
 160         lockdep_set_class(&fbc->lock, key);
 161         fbc->count = amount;
 162         fbc->counters = alloc_percpu_gfp(s32, gfp);
 163         if (!fbc->counters)
 164                 return -ENOMEM;
 165
 166         debug_percpu_counter_activate(fbc);
 167
 168 #ifdef CONFIG_HOTPLUG_CPU
 169         INIT_LIST_HEAD(&fbc->list);
 170         spin_lock_irqsave(&percpu_counters_lock, flags);
 171         list_add(&fbc->list, &percpu_counters);
 172         spin_unlock_irqrestore(&percpu_counters_lock, flags);
 173 #endif
 174         return 0;
 175 }
 176 EXPORT_SYMBOL(__percpu_counter_init);
 177
 178 void percpu_counter_destroy(struct percpu_counter *fbc)
 179 {
 180         unsigned long flags __maybe_unused;
 181
 182         if (!fbc->counters)
 183                 return;
 184
 185         debug_percpu_counter_deactivate(fbc);
 186
 187 #ifdef CONFIG_HOTPLUG_CPU
 188         spin_lock_irqsave(&percpu_counters_lock, flags);
 189         list_del(&fbc->list);
 190         spin_unlock_irqrestore(&percpu_counters_lock, flags);
 191 #endif
 192         free_percpu(fbc->counters);
 193         fbc->counters = NULL;
 194 }
 195 EXPORT_SYMBOL(percpu_counter_destroy);
 196
 197 int percpu_counter_batch __read_mostly = 32;
 198 EXPORT_SYMBOL(percpu_counter_batch);
 199
 200 static int compute_batch_value(unsigned int cpu)
 201 {
 202         int nr = num_online_cpus();
 203
 204         percpu_counter_batch = max(32, nr*2);
 205         return 0;
 206 }
 207
 208 static int percpu_counter_cpu_dead(unsigned int cpu)
 209 {
 210 #ifdef CONFIG_HOTPLUG_CPU
 211         struct percpu_counter *fbc;
 212
 213         compute_batch_value(cpu);
 214
 215         spin_lock_irq(&percpu_counters_lock);
 216         list_for_each_entry(fbc, &percpu_counters, list) {
 217                 s32 *pcount;
 218
 219                 raw_spin_lock(&fbc->lock);
 220                 pcount = per_cpu_ptr(fbc->counters, cpu);
 221                 fbc->count += *pcount;
 222                 *pcount = 0;
 223                 raw_spin_unlock(&fbc->lock);
 224         }
 225         spin_unlock_irq(&percpu_counters_lock);
 226 #endif
 227         return 0;
 228 }
 229
 230 /*
 231  * Compare counter against given value.
 232  * Return 1 if greater, 0 if equal and -1 if less
 233  */
 234 int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)
 235 {
 236         s64     count;
 237
 238         count = percpu_counter_read(fbc);
 239         /* Check to see if rough count will be sufficient for comparison */
 240         if (abs(count - rhs) > (batch * num_online_cpus())) {
 241                 if (count > rhs)
 242                         return 1;
 243                 else
 244                         return -1;
 245         }
 246         /* Need to use precise count */
 247         count = percpu_counter_sum(fbc);
 248         if (count > rhs)
 249                 return 1;
 250         else if (count < rhs)
 251                 return -1;
 252         else
 253                 return 0;
 254 }
 255 EXPORT_SYMBOL(__percpu_counter_compare);
 256
 257 static int __init percpu_counter_startup(void)
 258 {
 259         int ret;
 260
 261         ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "lib/percpu_cnt:online",
 262                                 compute_batch_value, NULL);
 263         WARN_ON(ret < 0);
 264         ret = cpuhp_setup_state_nocalls(CPUHP_PERCPU_CNT_DEAD,
 265                                         "lib/percpu_cnt:dead", NULL,
 266                                         percpu_counter_cpu_dead);
 267         WARN_ON(ret < 0);
 268         return 0;
 269 }
 270 module_init(percpu_counter_startup);