[PATCH v6 3/4] percpu_counter: add percpu_counter_sync()

From: Feng Tang <feng.tang@intel.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Matthew Wilcox <willy@infradead.org>,
	Mel Gorman <mgorman@suse.de>, Kees Cook <keescook@chromium.org>,
	Qian Cai <cai@lca.pw>, Dennis Zhou <dennis@kernel.org>,
	andi.kleen@intel.com, tim.c.chen@intel.com,
	dave.hansen@intel.com, ying.huang@intel.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Cc: Feng Tang <feng.tang@intel.com>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>
Subject: [PATCH v6 3/4] percpu_counter: add percpu_counter_sync()
Date: Fri, 10 Jul 2020 22:01:47 +0800	[thread overview]
Message-ID: <1594389708-60781-4-git-send-email-feng.tang@intel.com> (raw)
In-Reply-To: <1594389708-60781-1-git-send-email-feng.tang@intel.com>

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.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>
---
 include/linux/percpu_counter.h |  4 ++++
 lib/percpu_counter.c           | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 0a4f54d..01861ee 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -44,6 +44,7 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
 			      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_initialized(struct percpu_counter *fbc)
 	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)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index a66595b..a2345de 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -99,6 +99,25 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
 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()
  */
-- 
2.7.4

