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Commit 4a8c7bb5 authored by Nathan Zimmer's avatar Nathan Zimmer Committed by Linus Torvalds
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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: default avatarNathan Zimmer <nzimmer@sgi.com>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Acked-by: default avatarVlastimil 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8f235d1a

fs/hugetlbfs/inode.c

+1 −1
Original line number Diff line number Diff line
@@ -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

+1 −1
Original line number Diff line number Diff line
@@ -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

+17 −13
Original line number Diff line number Diff line
@@ -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 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start, 
	}

	if (new)

		sp_insert(sp, new);

	spin_unlock(&sp->lock);

	write_unlock(&sp->lock);

	ret = 0;



err_out:
@@ -2405,7 +2409,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start, 
	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