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inetpeer: seqlock optimization
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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>
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Eric Dumazet authored and David S. Miller committed Mar 4, 2011
1 parent d72751e commit 65e8354
Showing 1 changed file with 35 additions and 40 deletions.
75 changes: 35 additions & 40 deletions net/ipv4/inetpeer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
};

Expand Down Expand Up @@ -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.
*/
Expand All @@ -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) \
Expand All @@ -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; \
})
Expand Down Expand Up @@ -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; \
})
Expand All @@ -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 */
Expand All @@ -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 */
Expand All @@ -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 */
Expand All @@ -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 */
Expand Down Expand Up @@ -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,
Expand All @@ -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.
Expand All @@ -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);
Expand Down Expand Up @@ -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) {
Expand All @@ -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;
Expand All @@ -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. */
Expand Down

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