[107/163] percpu_counter: add percpu_counter_sync()

Commit Message

Andrew Morton Aug. 7, 2020, 6:23 a.m. UTC

From: Feng Tang <[e-mail]>
Subject: percpu_counter: add percpu_counter_sync()

percpu_counter's accuracy is related to its batch size.  For a
percpu_counter with a big batch, its deviation could be big, so when the
counter's batch is runtime changed to a smaller value for better accuracy,
there could also be requirment to reduce the big deviation.

So add a percpu-counter sync function to be run on each CPU.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Feng Tang <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Qian Cai <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Huang Ying <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: "K. Y. Srinivasan" <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Tim Chen <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/percpu_counter.h |    4 ++++
 lib/percpu_counter.c           |   19 +++++++++++++++++++
 2 files changed, 23 insertions(+)

Patch

--- a/include/linux/percpu_counter.h~percpu_counter-add-percpu_counter_sync
+++ a/include/linux/percpu_counter.h
@@ -44,6 +44,7 @@  void percpu_counter_add_batch(struct per
 			      s32 batch);
 s64 __percpu_counter_sum(struct percpu_counter *fbc);
 int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);
+void percpu_counter_sync(struct percpu_counter *fbc);
 
 static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
 {
@@ -172,6 +173,9 @@  static inline bool percpu_counter_initia
 	return true;
 }
 
+static inline void percpu_counter_sync(struct percpu_counter *fbc)
+{
+}
 #endif	/* CONFIG_SMP */
 
 static inline void percpu_counter_inc(struct percpu_counter *fbc)
--- a/lib/percpu_counter.c~percpu_counter-add-percpu_counter_sync
+++ a/lib/percpu_counter.c
@@ -99,6 +99,25 @@  void percpu_counter_add_batch(struct per
 EXPORT_SYMBOL(percpu_counter_add_batch);
 
 /*
+ * For percpu_counter with a big batch, the devication of its count could
+ * be big, and there is requirement to reduce the deviation, like when the
+ * counter's batch could be runtime decreased to get a better accuracy,
+ * which can be achieved by running this sync function on each CPU.
+ */
+void percpu_counter_sync(struct percpu_counter *fbc)
+{
+	unsigned long flags;
+	s64 count;
+
+	raw_spin_lock_irqsave(&fbc->lock, flags);
+	count = __this_cpu_read(*fbc->counters);
+	fbc->count += count;
+	__this_cpu_sub(*fbc->counters, count);
+	raw_spin_unlock_irqrestore(&fbc->lock, flags);
+}
+EXPORT_SYMBOL(percpu_counter_sync);
+
+/*
  * Add up all the per-cpu counts, return the result.  This is a more accurate
  * but much slower version of percpu_counter_read_positive()
  */