mm/mempolicy.c: convert the shared_policy lock to a rwlock

When running the SPECint_rate gcc on some very large boxes it was
noticed that the system was spending lots of time in
mpol_shared_policy_lookup().  The gamess benchmark can also show it and
is what I mostly used to chase down the issue since the setup for that I
found to be easier.

To be clear the binaries were on tmpfs because of disk I/O requirements.
We then used text replication to avoid icache misses and having all the
copies banging on the memory where the instruction code resides.  This
results in us hitting a bottleneck in mpol_shared_policy_lookup() since
lookup is serialised by the shared_policy lock.

I have only reproduced this on very large (3k+ cores) boxes.  The
problem starts showing up at just a few hundred ranks getting worse
until it threatens to livelock once it gets large enough.  For example
on the gamess benchmark at 128 ranks this area consumes only ~1% of
time, at 512 ranks it consumes nearly 13%, and at 2k ranks it is over
90%.

To alleviate the contention in this area I converted the spinlock to an
rwlock.  This allows a large number of lookups to happen simultaneously.
The results were quite good reducing this consumtion at max ranks to
around 2%.

[akpm@linux-foundation.org: tidy up code comments]
Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Nadia Yvette Chambers <nyc@holomorphy.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/hugetlbfs/inode.c

@@ -738,7 +738,7 @@ static struct inode *hugetlbfs_get_inode(struct super_block *sb,
 		/*
 		 * The policy is initialized here even if we are creating a
 		 * private inode because initialization simply creates an
-		 * an empty rb tree and calls spin_lock_init(), later when we
+		 * an empty rb tree and calls rwlock_init(), later when we
 		 * call mpol_free_shared_policy() it will just return because
 		 * the rb tree will still be empty.
 		 */

include/linux/mempolicy.h

@@ -122,7 +122,7 @@ struct sp_node {
 
 struct shared_policy {
 	struct rb_root root;
-	spinlock_t lock;
+	rwlock_t lock;
 };
 
 int vma_dup_policy(struct vm_area_struct *src, struct vm_area_struct *dst);

mm/mempolicy.c

@@ -2142,12 +2142,14 @@ bool __mpol_equal(struct mempolicy *a, struct mempolicy *b)
  *
  * Remember policies even when nobody has shared memory mapped.
  * The policies are kept in Red-Black tree linked from the inode.
- * They are protected by the sp->lock spinlock, which should be held
+ * They are protected by the sp->lock rwlock, which should be held
  * for any accesses to the tree.
  */
 
-/* lookup first element intersecting start-end */
+/*
-/* Caller holds sp->lock */
+ * lookup first element intersecting start-end.  Caller holds sp->lock for
+ * reading or for writing
+ */
 static struct sp_node *
 sp_lookup(struct shared_policy *sp, unsigned long start, unsigned long end)
 {
@@ -2178,8 +2180,10 @@ sp_lookup(struct shared_policy *sp, unsigned long start, unsigned long end)
 	return rb_entry(n, struct sp_node, nd);
 }
 
-/* Insert a new shared policy into the list. */
+/*
-/* Caller holds sp->lock */
+ * Insert a new shared policy into the list.  Caller holds sp->lock for
+ * writing.
+ */
 static void sp_insert(struct shared_policy *sp, struct sp_node *new)
 {
 	struct rb_node **p = &sp->root.rb_node;
@@ -2211,13 +2215,13 @@ mpol_shared_policy_lookup(struct shared_policy *sp, unsigned long idx)
 
 	if (!sp->root.rb_node)
 		return NULL;
-	spin_lock(&sp->lock);
+	read_lock(&sp->lock);
 	sn = sp_lookup(sp, idx, idx+1);
 	if (sn) {
 		mpol_get(sn->policy);
 		pol = sn->policy;
 	}
-	spin_unlock(&sp->lock);
+	read_unlock(&sp->lock);
 	return pol;
 }
 
@@ -2360,7 +2364,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
 	int ret = 0;
 
 restart:
-	spin_lock(&sp->lock);
+	write_lock(&sp->lock);
 	n = sp_lookup(sp, start, end);
 	/* Take care of old policies in the same range. */
 	while (n && n->start < end) {
@@ -2393,7 +2397,7 @@ restart:
 	}
 	if (new)
 		sp_insert(sp, new);
-	spin_unlock(&sp->lock);
+	write_unlock(&sp->lock);
 	ret = 0;
 
 err_out:
@@ -2405,7 +2409,7 @@ err_out:
 	return ret;
 
 alloc_new:
-	spin_unlock(&sp->lock);
+	write_unlock(&sp->lock);
 	ret = -ENOMEM;
 	n_new = kmem_cache_alloc(sn_cache, GFP_KERNEL);
 	if (!n_new)
@@ -2431,7 +2435,7 @@ void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol)
 	int ret;
 
 	sp->root = RB_ROOT;		/* empty tree == default mempolicy */
-	spin_lock_init(&sp->lock);
+	rwlock_init(&sp->lock);
 
 	if (mpol) {
 		struct vm_area_struct pvma;
@@ -2497,14 +2501,14 @@ void mpol_free_shared_policy(struct shared_policy *p)
 
 	if (!p->root.rb_node)
 		return;
-	spin_lock(&p->lock);
+	write_lock(&p->lock);
 	next = rb_first(&p->root);
 	while (next) {
 		n = rb_entry(next, struct sp_node, nd);
 		next = rb_next(&n->nd);
 		sp_delete(p, n);
 	}
-	spin_unlock(&p->lock);
+	write_unlock(&p->lock);
 }
 
 #ifdef CONFIG_NUMA_BALANCING