[Devel] Re: [PATCH] memory.min_usage (seqlock for res_counter)

[KAMEZAWA Hiroyuki]

This is seqlock version res_counter.
Maybe this this will reduce # of spin_lock.

Pavel-san, How about this ?

-Kame
==
resource counter's spinlock can be seqlock. (res_counter doesn't include
any pointers.)

And it will be good to avoid atomic ops if we can when we just checks
value.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>




 include/linux/res_counter.h |   24 +++++++-----------------
 kernel/res_counter.c        |   14 +++++++-------
 2 files changed, 14 insertions(+), 24 deletions(-)

Index: linux-2.6.24-rc3-mm2/include/linux/res_counter.h
===================================================================
--- linux-2.6.24-rc3-mm2.orig/include/linux/res_counter.h
+++ linux-2.6.24-rc3-mm2/include/linux/res_counter.h
@@ -12,6 +12,7 @@
  */
 
 #include <linux/cgroup.h>
+#include <linux/seqlock.h>
 
 /*
  * The core object. the cgroup that wishes to account for some
@@ -36,7 +37,7 @@ struct res_counter {
 	 * the lock to protect all of the above.
 	 * the routines below consider this to be IRQ-safe
 	 */
-	spinlock_t lock;
+	seqlock_t lock;
 };
 
 /*
@@ -101,27 +102,17 @@ int res_counter_charge(struct res_counte
 void res_counter_uncharge_locked(struct res_counter *counter, unsigned long val);
 void res_counter_uncharge(struct res_counter *counter, unsigned long val);
 
-static inline bool res_counter_limit_check_locked(struct res_counter *cnt)
-{
-	if (cnt->usage < cnt->limit)
-		return true;
-
-	return false;
-}
-
-/*
- * Helper function to detect if the cgroup is within it's limit or
- * not. It's currently called from cgroup_rss_prepare()
- */
 static inline bool res_counter_check_under_limit(struct res_counter *cnt)
 {
 	bool ret;
-	unsigned long flags;
+	unsigned long seq;
 
-	spin_lock_irqsave(&cnt->lock, flags);
-	ret = res_counter_limit_check_locked(cnt);
-	spin_unlock_irqrestore(&cnt->lock, flags);
+	do {
+		seq = read_seqbegin(&cnt->lock);
+		ret = (cnt->usage < cnt->limit);
+	} while (read_seqretry(&cnt->lock, seq));
 	return ret;
 }
 
+
 #endif
Index: linux-2.6.24-rc3-mm2/kernel/res_counter.c
===================================================================
--- linux-2.6.24-rc3-mm2.orig/kernel/res_counter.c
+++ linux-2.6.24-rc3-mm2/kernel/res_counter.c
@@ -15,7 +15,7 @@
 
 void res_counter_init(struct res_counter *counter)
 {
-	spin_lock_init(&counter->lock);
+	seqlock_init(&counter->lock);
 	counter->limit = (unsigned long long)LLONG_MAX;
 }
 
@@ -35,9 +35,9 @@ int res_counter_charge(struct res_counte
 	int ret;
 	unsigned long flags;
 
-	spin_lock_irqsave(&counter->lock, flags);
+	write_seqlock_irqsave(&counter->lock, flags);
 	ret = res_counter_charge_locked(counter, val);
-	spin_unlock_irqrestore(&counter->lock, flags);
+	write_sequnlock_irqrestore(&counter->lock, flags);
 	return ret;
 }
 
@@ -53,9 +53,9 @@ void res_counter_uncharge(struct res_cou
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&counter->lock, flags);
+	write_seqlock_irqsave(&counter->lock, flags);
 	res_counter_uncharge_locked(counter, val);
-	spin_unlock_irqrestore(&counter->lock, flags);
+	write_sequnlock_irqrestore(&counter->lock, flags);
 }
 
 
@@ -122,10 +122,10 @@ ssize_t res_counter_write(struct res_cou
 		if (*end != '\0')
 			goto out_free;
 	}
-	spin_lock_irqsave(&counter->lock, flags);
+	write_seqlock_irqsave(&counter->lock, flags);
 	val = res_counter_member(counter, member);
 	*val = tmp;
-	spin_unlock_irqrestore(&counter->lock, flags);
+	write_sequnlock_irqrestore(&counter->lock, flags);
 	ret = nbytes;
 out_free:
 	kfree(buf);

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