[RFC 6/7] Add min and max statistics to percpu_counter

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

Because percpu counters can accumulate their per-cpu sums
over time outside of the control of the callers, we need
this patch that updates the maximum values when that
happens.

I am adding a mininum value as well for conistency, due to
the signed nature of the percpu_counters.

However, I am not sure this will be of general use, and might
be yet another indication that we need to duplicate those
structures...

Signed-off-by: Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
---
 include/linux/percpu_counter.h |    2 ++
 include/linux/res_counter.h    |    6 +-----
 kernel/res_counter.c           |    6 +++---
 lib/percpu_counter.c           |    4 ++++
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 8310548..639d2d5 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -18,6 +18,8 @@
 struct percpu_counter {
 	raw_spinlock_t lock;
 	s64 count;
+	s64 max;
+	s64 min;
 #ifdef CONFIG_HOTPLUG_CPU
 	struct list_head list;	/* All percpu_counters are on a list */
 #endif
diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
index 8c1c20e..3527827 100644
--- a/include/linux/res_counter.h
+++ b/include/linux/res_counter.h
@@ -27,10 +27,6 @@ struct res_counter {
 	 */
 	struct percpu_counter usage_pcp;
 	/*
-	 * the maximal value of the usage from the counter creation
-	 */
-	unsigned long long max_usage;
-	/*
 	 * the limit that usage cannot exceed
 	 */
 	unsigned long long limit;
@@ -178,7 +174,7 @@ static inline void res_counter_reset_max(struct res_counter *cnt)
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&cnt->usage_pcp.lock, flags);
-	cnt->max_usage = __percpu_counter_sum_locked(&cnt->usage_pcp);
+	cnt->usage_pcp.max = __percpu_counter_sum_locked(&cnt->usage_pcp);
 	raw_spin_unlock_irqrestore(&cnt->usage_pcp.lock, flags);
 }
 
diff --git a/kernel/res_counter.c b/kernel/res_counter.c
index 8a99943..7b05208 100644
--- a/kernel/res_counter.c
+++ b/kernel/res_counter.c
@@ -61,8 +61,8 @@ int __res_counter_add(struct res_counter *c, long val, bool fail)
 
 	c->usage_pcp.count += val;
 
-	if (usage > c->max_usage)
-		c->max_usage = usage;
+	if (usage > c->usage_pcp.max)
+		c->usage_pcp.max = usage;
 
 out:
 	raw_spin_unlock(&c->usage_pcp.lock);
@@ -164,7 +164,7 @@ res_counter_member(struct res_counter *counter, int member)
 {
 	switch (member) {
 	case RES_MAX_USAGE:
-		return &counter->max_usage;
+		return &counter->usage_pcp.max;
 	case RES_LIMIT:
 		return &counter->limit;
 	case RES_FAILCNT:
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 0b6a672..6dff70b 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -81,6 +81,10 @@ void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
 		raw_spin_lock(&fbc->lock);
 		fbc->count += count;
 		__this_cpu_write(*fbc->counters, 0);
+		if (fbc->count > fbc->max)
+			fbc->max = fbc->count;
+		if (fbc->count < fbc->min)
+			fbc->min = fbc->count;
 		raw_spin_unlock(&fbc->lock);
 	} else {
 		__this_cpu_write(*fbc->counters, count);
-- 
1.7.4.1