Commit 4a8c7bb5 authored by Nathan Zimmer's avatar Nathan Zimmer Committed by Linus Torvalds

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

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%.

[ tidy up code comments]
Signed-off-by: default avatarNathan Zimmer <>
Acked-by: default avatarDavid Rientjes <>
Acked-by: default avatarVlastimil Babka <>
Cc: Nadia Yvette Chambers <>
Cc: Naoya Horiguchi <>
Cc: Mel Gorman <>
Cc: "Aneesh Kumar K.V" <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 8f235d1a
......@@ -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.
......@@ -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);
......@@ -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;
sn = sp_lookup(sp, idx, idx+1);
if (sn) {
pol = sn->policy;
return pol;
......@@ -2360,7 +2364,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
int ret = 0;
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);
ret = 0;
......@@ -2405,7 +2409,7 @@ err_out:
return ret;
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 */
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)
next = rb_first(&p->root);
while (next) {
n = rb_entry(next, struct sp_node, nd);
next = rb_next(&n->nd);
sp_delete(p, n);
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