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 a easier.
To be clear the binaries were on tmpfs because of disk I/O reqruirements. 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 bottle neck 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 on this area I converted the spinslock to a rwlock. This allows the large number of lookups to happen simultaneously. The results were quite good reducing this to consumtion at max ranks to around 2%. Acked-by: David Rientjes <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Nadia Yvette Chambers <[email protected]> Cc: Naoya Horiguchi <[email protected]> Cc: Mel Gorman <[email protected]> Cc: "Aneesh Kumar K.V" <[email protected]> Cc: [email protected] Cc: [email protected] Signed-off-by: Nathan Zimmer <[email protected]> --- fs/hugetlbfs/inode.c | 2 +- include/linux/mempolicy.h | 2 +- mm/mempolicy.c | 20 ++++++++++---------- 3 files changed, 12 insertions(+), 12 deletions(-) diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c index 316adb9..ab7b155 100644 --- a/fs/hugetlbfs/inode.c +++ b/fs/hugetlbfs/inode.c @@ -739,7 +739,7 @@ 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. */ diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h index 3d385c8..2696c1f 100644 --- a/include/linux/mempolicy.h +++ b/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); diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 87a1779..197d917 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -2142,7 +2142,7 @@ 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. */ @@ -2179,7 +2179,7 @@ sp_lookup(struct shared_policy *sp, unsigned long start, unsigned long end) } /* Insert a new shared policy into the list. */ -/* Caller holds sp->lock */ +/* Caller holds the write of sp->lock */ static void sp_insert(struct shared_policy *sp, struct sp_node *new) { struct rb_node **p = &sp->root.rb_node; @@ -2211,13 +2211,13 @@ 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 +2360,7 @@ 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 +2393,7 @@ restart: } if (new) sp_insert(sp, new); - spin_unlock(&sp->lock); + write_unlock(&sp->lock); ret = 0; err_out: @@ -2405,7 +2405,7 @@ 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 +2431,7 @@ 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 +2497,14 @@ 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 -- 1.8.2.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
