percpu_counter: add percpu_counter_sum_all interface

The percpu_counter is used for scenarios where performance is more important than the accuracy. For percpu_counter users, who want more accurate information in their slowpath, percpu_counter_sum is provided which traverses all the online CPUs to accumulate the data. The reason it only needs to traverse online CPUs is because percpu_counter does implement CPU offline callback which syncs the local data of the offlined CPU. However there is a small race window between the online CPUs traversal of percpu_counter_sum and the CPU offline callback. The offline callback has to traverse all the percpu_counters on the system to flush the CPU local data which can be a lot. During that time, the CPU which is going offline has already been published as offline to all the readers. So, as the offline callback is running, percpu_counter_sum can be called for one counter which has some state on the CPU going offline. Since percpu_counter_sum only traverses online CPUs, it will skip that specific CPU and the offline callback might not have flushed the state for that specific percpu_counter on that offlined CPU. Normally this is not an issue because percpu_counter users can deal with some inaccuracy for small time window. However a new user i.e. mm_struct on the cleanup path wants to check the exact state of the percpu_counter through check_mm(). For such users, this patch introduces percpu_counter_sum_all() which traverses all possible CPUs and it is used in fork.c:check_mm() to avoid the potential race. This issue is exposed by the later patch "mm: convert mm's rss stats into percpu_counter".

Shakeel Butt committed 4 years ago

Commit f6890541 parent f1a79413 files · +34 −6Browse files

Unified · Split

3 files

include/linux/percpu_counter.h +6 −0
kernel/fork.c +5 −0
lib/percpu_counter.c +23 −6

include/linux/percpu_counter.h+6−0

 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
 			      s32 batch);
 s64 __percpu_counter_sum(struct percpu_counter *fbc);
+s64 percpu_counter_sum_all(struct percpu_counter *fbc);
 int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);
 void percpu_counter_sync(struct percpu_counter *fbc);
 	return percpu_counter_read(fbc);
 }
+static inline s64 percpu_counter_sum_all(struct percpu_counter *fbc)
+{
+	return percpu_counter_read(fbc);
+}
 static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
 {
 	return true;

kernel/fork.c+5−0

 	for (i = 0; i < NR_MM_COUNTERS; i++) {
 		long x = percpu_counter_sum(&mm->rss_stat[i]);
+		if (likely(!x))
+			continue;
+		/* Making sure this is not due to race with CPU offlining. */
+		x = percpu_counter_sum_all(&mm->rss_stat[i]);
 		if (unlikely(x))
 			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
 				 mm, resident_page_types[i], x);

lib/percpu_counter.c+23−6

 }
 EXPORT_SYMBOL(percpu_counter_sync);
-/*
+static s64 __percpu_counter_sum_mask(struct percpu_counter *fbc,
- * Add up all the per-cpu counts, return the result.  This is a more accurate
+			      const struct cpumask *cpu_mask)
- * but much slower version of percpu_counter_read_positive()
- */
-s64 __percpu_counter_sum(struct percpu_counter *fbc)
 {
 	s64 ret;
 	int cpu;
 	raw_spin_lock_irqsave(&fbc->lock, flags);
 	ret = fbc->count;
-	for_each_online_cpu(cpu) {
+	for_each_cpu(cpu, cpu_mask) {
 		s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
 		ret += *pcount;
 	}
 	raw_spin_unlock_irqrestore(&fbc->lock, flags);
 	return ret;
 }
+/*
+ * Add up all the per-cpu counts, return the result.  This is a more accurate
+ * but much slower version of percpu_counter_read_positive()
+ */
+s64 __percpu_counter_sum(struct percpu_counter *fbc)
+{
+	return __percpu_counter_sum_mask(fbc, cpu_online_mask);
+}
 EXPORT_SYMBOL(__percpu_counter_sum);
+/*
+ * This is slower version of percpu_counter_sum as it traverses all possible
+ * cpus. Use this only in the cases where accurate data is needed in the
+ * presense of CPUs getting offlined.
+ */
+s64 percpu_counter_sum_all(struct percpu_counter *fbc)
+{
+	return __percpu_counter_sum_mask(fbc, cpu_possible_mask);
+}
+EXPORT_SYMBOL(percpu_counter_sum_all);
 int __percpu_counter_init(struct percpu_counter *fbc, s64 amount, gfp_t gfp,
 			  struct lock_class_key *key)
 {