inetpeer: seqlock optimization

David noticed :

------------------
Eric, I was profiling the non-routing-cache case and something that
stuck out is the case of calling inet_getpeer() with create==0.

If an entry is not found, we have to redo the lookup under a spinlock
to make certain that a concurrent writer rebalancing the tree does
not "hide" an existing entry from us.

This makes the case of a create==0 lookup for a not-present entry
really expensive.  It is on the order of 600 cpu cycles on my
Niagara2.

I added a hack to not do the relookup under the lock when create==0
and it now costs less than 300 cycles.

This is now a pretty common operation with the way we handle COW'd
metrics, so I think it's definitely worth optimizing.
-----------------

One solution is to use a seqlock instead of a spinlock to protect struct
inet_peer_base.

After a failed avl tree lookup, we can easily detect if a writer did
some changes during our lookup. Taking the lock and redo the lookup is
only necessary in this case.

Note: Add one private rcu_deref_locked() macro to place in one spot the
access to spinlock included in seqlock.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 35 insertions, 40 deletions

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index 48f8d4592ccd..f604ffdbea27 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -81,19 +81,19 @@ static const struct inet_peer peer_fake_node = {
 
 struct inet_peer_base {
         struct inet_peer __rcu *root;
-        spinlock_t      lock;
+        seqlock_t       lock;
         int             total;
 };
 
 static struct inet_peer_base v4_peers = {
         .root           = peer_avl_empty_rcu,
-        .lock           = __SPIN_LOCK_UNLOCKED(v4_peers.lock),
+        .lock           = __SEQLOCK_UNLOCKED(v4_peers.lock),
         .total          = 0,
 };
 
 static struct inet_peer_base v6_peers = {
         .root           = peer_avl_empty_rcu,
-        .lock           = __SPIN_LOCK_UNLOCKED(v6_peers.lock),
+        .lock           = __SEQLOCK_UNLOCKED(v6_peers.lock),
         .total          = 0,
 };
 
@@ -177,6 +177,9 @@ static int addr_compare(const struct inetpeer_addr *a,
         return 0;
 }
 
+#define rcu_deref_locked(X, BASE)                               \
+        rcu_dereference_protected(X, lockdep_is_held(&(BASE)->lock.lock))
+
 /*
  * Called with local BH disabled and the pool lock held.
  */
@@ -187,8 +190,7 @@ static int addr_compare(const struct inetpeer_addr *a,
                                                                 \
         stackptr = _stack;                                      \
         *stackptr++ = &_base->root;                             \
-        for (u = rcu_dereference_protected(_base->root,         \
-                        lockdep_is_held(&_base->lock));         \
+        for (u = rcu_deref_locked(_base->root, _base);          \
              u != peer_avl_empty; ) {                           \
                 int cmp = addr_compare(_daddr, &u->daddr);      \
                 if (cmp == 0)                                   \
@@ -198,8 +200,7 @@ static int addr_compare(const struct inetpeer_addr *a,
                 else                                            \
                         v = &u->avl_right;                      \
                 *stackptr++ = v;                                \
-                u = rcu_dereference_protected(*v,               \
-                        lockdep_is_held(&_base->lock));         \
+                u = rcu_deref_locked(*v, _base);                \
         }                                                       \
         u;                                                      \
 })
@@ -246,13 +247,11 @@ static struct inet_peer *lookup_rcu_bh(const struct inetpeer_addr *daddr,
         struct inet_peer __rcu **v;                             \
         *stackptr++ = &start->avl_left;                         \
         v = &start->avl_left;                                   \
-        for (u = rcu_dereference_protected(*v,                  \
-                        lockdep_is_held(&base->lock));          \
+        for (u = rcu_deref_locked(*v, base);                    \
              u->avl_right != peer_avl_empty_rcu; ) {            \
                 v = &u->avl_right;                              \
                 *stackptr++ = v;                                \
-                u = rcu_dereference_protected(*v,               \
-                        lockdep_is_held(&base->lock));          \
+                u = rcu_deref_locked(*v, base);                 \
         }                                                       \
         u;                                                      \
 })
@@ -271,21 +270,16 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
 
         while (stackend > stack) {
                 nodep = *--stackend;
-                node = rcu_dereference_protected(*nodep,
-                                lockdep_is_held(&base->lock));
-                l = rcu_dereference_protected(node->avl_left,
-                                lockdep_is_held(&base->lock));
-                r = rcu_dereference_protected(node->avl_right,
-                                lockdep_is_held(&base->lock));
+                node = rcu_deref_locked(*nodep, base);
+                l = rcu_deref_locked(node->avl_left, base);
+                r = rcu_deref_locked(node->avl_right, base);
                 lh = node_height(l);
                 rh = node_height(r);
                 if (lh > rh + 1) { /* l: RH+2 */
                         struct inet_peer *ll, *lr, *lrl, *lrr;
                         int lrh;
-                        ll = rcu_dereference_protected(l->avl_left,
-                                lockdep_is_held(&base->lock));
-                        lr = rcu_dereference_protected(l->avl_right,
-                                lockdep_is_held(&base->lock));
+                        ll = rcu_deref_locked(l->avl_left, base);
+                        lr = rcu_deref_locked(l->avl_right, base);
                         lrh = node_height(lr);
                         if (lrh <= node_height(ll)) {   /* ll: RH+1 */
                                 RCU_INIT_POINTER(node->avl_left, lr);   /* lr: RH or RH+1 */
@@ -296,10 +290,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
                                 l->avl_height = node->avl_height + 1;
                                 RCU_INIT_POINTER(*nodep, l);
                         } else { /* ll: RH, lr: RH+1 */
-                                lrl = rcu_dereference_protected(lr->avl_left,
-                                        lockdep_is_held(&base->lock));  /* lrl: RH or RH-1 */
-                                lrr = rcu_dereference_protected(lr->avl_right,
-                                        lockdep_is_held(&base->lock));  /* lrr: RH or RH-1 */
+                                lrl = rcu_deref_locked(lr->avl_left, base);/* lrl: RH or RH-1 */
+                                lrr = rcu_deref_locked(lr->avl_right, base);/* lrr: RH or RH-1 */
                                 RCU_INIT_POINTER(node->avl_left, lrr);  /* lrr: RH or RH-1 */
                                 RCU_INIT_POINTER(node->avl_right, r);   /* r: RH */
                                 node->avl_height = rh + 1; /* node: RH+1 */
@@ -314,10 +306,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
                 } else if (rh > lh + 1) { /* r: LH+2 */
                         struct inet_peer *rr, *rl, *rlr, *rll;
                         int rlh;
-                        rr = rcu_dereference_protected(r->avl_right,
-                                lockdep_is_held(&base->lock));
-                        rl = rcu_dereference_protected(r->avl_left,
-                                lockdep_is_held(&base->lock));
+                        rr = rcu_deref_locked(r->avl_right, base);
+                        rl = rcu_deref_locked(r->avl_left, base);
                         rlh = node_height(rl);
                         if (rlh <= node_height(rr)) {   /* rr: LH+1 */
                                 RCU_INIT_POINTER(node->avl_right, rl);  /* rl: LH or LH+1 */
@@ -328,10 +318,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
                                 r->avl_height = node->avl_height + 1;
                                 RCU_INIT_POINTER(*nodep, r);
                         } else { /* rr: RH, rl: RH+1 */
-                                rlr = rcu_dereference_protected(rl->avl_right,
-                                        lockdep_is_held(&base->lock));  /* rlr: LH or LH-1 */
-                                rll = rcu_dereference_protected(rl->avl_left,
-                                        lockdep_is_held(&base->lock));  /* rll: LH or LH-1 */
+                                rlr = rcu_deref_locked(rl->avl_right, base);/* rlr: LH or LH-1 */
+                                rll = rcu_deref_locked(rl->avl_left, base);/* rll: LH or LH-1 */
                                 RCU_INIT_POINTER(node->avl_right, rll); /* rll: LH or LH-1 */
                                 RCU_INIT_POINTER(node->avl_left, l);    /* l: LH */
                                 node->avl_height = lh + 1; /* node: LH+1 */
@@ -372,7 +360,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
 
         do_free = 0;
 
-        spin_lock_bh(&base->lock);
+        write_seqlock_bh(&base->lock);
         /* Check the reference counter.  It was artificially incremented by 1
          * in cleanup() function to prevent sudden disappearing.  If we can
          * atomically (because of lockless readers) take this last reference,
@@ -392,8 +380,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
                         /* look for a node to insert instead of p */
                         struct inet_peer *t;
                         t = lookup_rightempty(p, base);
-                        BUG_ON(rcu_dereference_protected(*stackptr[-1],
-                                        lockdep_is_held(&base->lock)) != t);
+                        BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
                         **--stackptr = t->avl_left;
                         /* t is removed, t->daddr > x->daddr for any
                          * x in p->avl_left subtree.
@@ -409,7 +396,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
                 base->total--;
                 do_free = 1;
         }
-        spin_unlock_bh(&base->lock);
+        write_sequnlock_bh(&base->lock);
 
         if (do_free)
                 call_rcu_bh(&p->rcu, inetpeer_free_rcu);
@@ -477,12 +464,16 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
         struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
         struct inet_peer_base *base = family_to_base(daddr->family);
         struct inet_peer *p;
+        unsigned int sequence;
+        int invalidated;
 
         /* Look up for the address quickly, lockless.
          * Because of a concurrent writer, we might not find an existing entry.
          */
         rcu_read_lock_bh();
+        sequence = read_seqbegin(&base->lock);
         p = lookup_rcu_bh(daddr, base);
+        invalidated = read_seqretry(&base->lock, sequence);
         rcu_read_unlock_bh();
 
         if (p) {
@@ -493,14 +484,18 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
                 return p;
         }
 
+        /* If no writer did a change during our lookup, we can return early. */
+        if (!create && !invalidated)
+                return NULL;
+
         /* retry an exact lookup, taking the lock before.
          * At least, nodes should be hot in our cache.
          */
-        spin_lock_bh(&base->lock);
+        write_seqlock_bh(&base->lock);
         p = lookup(daddr, stack, base);
         if (p != peer_avl_empty) {
                 atomic_inc(&p->refcnt);
-                spin_unlock_bh(&base->lock);
+                write_sequnlock_bh(&base->lock);
                 /* Remove the entry from unused list if it was there. */
                 unlink_from_unused(p);
                 return p;
@@ -524,7 +519,7 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
                 link_to_pool(p, base);
                 base->total++;
         }
-        spin_unlock_bh(&base->lock);
+        write_sequnlock_bh(&base->lock);
 
         if (base->total >= inet_peer_threshold)
                 /* Remove one less-recently-used entry. */