diff mbox

inetpeer: optimizations

Message ID 4B1B6F87.6050201@gmail.com
State Deferred, archived
Delegated to: David Miller
Headers show

Commit Message

Eric Dumazet Dec. 6, 2009, 8:47 a.m. UTC 
Jarek Poplawski a écrit :
> Eric Dumazet wrote, On 12/05/2009 01:11 PM:
> 
>> - Use atomic_dec_and_test() in inet_putpeer()
> 
> atomic_dec_and_lock()?

Yes :)

> 
>>   This takes/dirties the lock only if necessary.
> ...
> 
>>  void inet_putpeer(struct inet_peer *p)
>>  {
>> -	spin_lock_bh(&inet_peer_unused_lock);
>> -	if (atomic_dec_and_test(&p->refcnt)) {
>> -		list_add_tail(&p->unused, &unused_peers);
>> +	local_bh_disable();
>> +	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
> 
> Why not:
> 	if (atomic_dec_and_test(&p->refcnt)) {
> 		spin_lock_bh(&inet_peer_unused_lock);
> 		...

Because we have to take the lock before doing the final 1 -> 0 refcount transition.

(Another thread could do the 0 -> 1 transition)

I'll cook a followup patch to also avoid taking the lock in the  1+ -> 2+ transitions.

Thanks

[PATCH] inetpeer: optimizations

- Use atomic_dec_and_lock() in inet_putpeer()
  This takes/dirties the lock only if necessary.

- Group fields together, since they currently are in BSS and DATA section,
  we have to dirty two cache lines instead of one.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 net/ipv4/inetpeer.c |   78 ++++++++++++++++++++++++------------------
 1 file changed, 45 insertions(+), 33 deletions(-)


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Comments

Jarek Poplawski Dec. 6, 2009, 2:18 p.m. UTC | #1 
On Sun, Dec 06, 2009 at 09:47:03AM +0100, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > Eric Dumazet wrote, On 12/05/2009 01:11 PM:
> >>  void inet_putpeer(struct inet_peer *p)
> >>  {
> >> -	spin_lock_bh(&inet_peer_unused_lock);
> >> -	if (atomic_dec_and_test(&p->refcnt)) {
> >> -		list_add_tail(&p->unused, &unused_peers);
> >> +	local_bh_disable();
> >> +	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
> > 
> > Why not:
> > 	if (atomic_dec_and_test(&p->refcnt)) {
> > 		spin_lock_bh(&inet_peer_unused_lock);
> > 		...
> 
> Because we have to take the lock before doing the final 1 -> 0 refcount transition.
> 
> (Another thread could do the 0 -> 1 transition)
> 
> I'll cook a followup patch to also avoid taking the lock in the  1+ -> 2+ transitions.

I see... So it's this concept of atomic refcounts with locking, which
I can't get used to. Anyway, since local_bh_disable/enable() are more
than one or two asm instructions, and this all is about optimization,
it seems to me it's worth to avoid it with one of these:
a) additional atomic test under the lock after unlocked
   atomic_dec_and_test(),
b) implementing atomic_dec_and_lock_bh(),
c) if there are are problems with b), open code it here.

Thanks,
Jarek P.
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Jarek Poplawski Dec. 6, 2009, 6:22 p.m. UTC | #2 
On Sun, Dec 06, 2009 at 09:47:03AM +0100, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > Eric Dumazet wrote, On 12/05/2009 01:11 PM:
> > 
> >> - Use atomic_dec_and_test() in inet_putpeer()
> > 
> > atomic_dec_and_lock()?
> 
> Yes :)
> 
> > 
> >>   This takes/dirties the lock only if necessary.
> > ...
> > 
> >>  void inet_putpeer(struct inet_peer *p)
> >>  {
> >> -	spin_lock_bh(&inet_peer_unused_lock);
> >> -	if (atomic_dec_and_test(&p->refcnt)) {
> >> -		list_add_tail(&p->unused, &unused_peers);
> >> +	local_bh_disable();
> >> +	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
> > 
> > Why not:
> > 	if (atomic_dec_and_test(&p->refcnt)) {
> > 		spin_lock_bh(&inet_peer_unused_lock);
> > 		...
> 
> Because we have to take the lock before doing the final 1 -> 0 refcount transition.
> 
> (Another thread could do the 0 -> 1 transition)

AFAICS this lock here can only to prevent double linking to the
unused_peers list during such transitions. If so, it could be replaced
with the list_empty(&p->unused) test before list_add_tail(), and
atomic_dec_test() without the lock would be enough (unless I miss
something ;-).

Jarek P.
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Jarek Poplawski Dec. 6, 2009, 6:53 p.m. UTC | #3 
On Sun, Dec 06, 2009 at 07:22:10PM +0100, Jarek Poplawski wrote:
> AFAICS this lock here can only to prevent double linking to the
> unused_peers list during such transitions. If so, it could be replaced
> with the list_empty(&p->unused) test before list_add_tail(), and
> atomic_dec_test() without the lock would be enough (unless I miss
> something ;-).

Hmm... But I missed something: the last atomic_dec() should be done
under the lock yet, so let's forget it.

Jarek P.
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Eric Dumazet Dec. 6, 2009, 6:58 p.m. UTC | #4 
Jarek Poplawski a écrit :
> On Sun, Dec 06, 2009 at 09:47:03AM +0100, Eric Dumazet wrote:
>> Jarek Poplawski a écrit :
>>> Eric Dumazet wrote, On 12/05/2009 01:11 PM:
>>>
>>>> - Use atomic_dec_and_test() in inet_putpeer()
>>> atomic_dec_and_lock()?
>> Yes :)
>>
>>>>   This takes/dirties the lock only if necessary.
>>> ...
>>>
>>>>  void inet_putpeer(struct inet_peer *p)
>>>>  {
>>>> -	spin_lock_bh(&inet_peer_unused_lock);
>>>> -	if (atomic_dec_and_test(&p->refcnt)) {
>>>> -		list_add_tail(&p->unused, &unused_peers);
>>>> +	local_bh_disable();
>>>> +	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
>>> Why not:
>>> 	if (atomic_dec_and_test(&p->refcnt)) {
>>> 		spin_lock_bh(&inet_peer_unused_lock);
>>> 		...
>> Because we have to take the lock before doing the final 1 -> 0 refcount transition.
>>
>> (Another thread could do the 0 -> 1 transition)
> 
> AFAICS this lock here can only to prevent double linking to the
> unused_peers list during such transitions. If so, it could be replaced
> with the list_empty(&p->unused) test before list_add_tail(), and
> atomic_dec_test() without the lock would be enough (unless I miss
> something ;-).
> 

Yes, you miss something. We are not working on a true reference count variable.
(p is referenced in avl tree but there is no +1 count for this reference)

Its more a usecount one, and p usecount can be 0 but p still in avl tree.

Even if we are the thread (A) doing 1 -> 0 transition, other thread (B)
can find p and perform the opposite 0 -> 1 transition.

If (B) tries to unlink p before (A), it finds p already unlinked.

Then (A) links into unused list, while refcnt is still 1

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Jarek Poplawski Dec. 6, 2009, 8:18 p.m. UTC | #5 
On Sun, Dec 06, 2009 at 07:58:07PM +0100, Eric Dumazet wrote:
> Yes, you miss something. We are not working on a true reference count variable.
> (p is referenced in avl tree but there is no +1 count for this reference)
> 
> Its more a usecount one, and p usecount can be 0 but p still in avl tree.
> 
> Even if we are the thread (A) doing 1 -> 0 transition, other thread (B)
> can find p and perform the opposite 0 -> 1 transition.
> 
> If (B) tries to unlink p before (A), it finds p already unlinked.
> 
> Then (A) links into unused list, while refcnt is still 1

This last thing can happen now too, but as I wrote, my idea was wrong,
nevertheless.

Jarek P.
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diff mbox

Patch

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index 6bcfe52..4125ef9 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -76,11 +76,17 @@  static struct inet_peer peer_fake_node = {
 	.avl_height	= 0
 };
 #define peer_avl_empty (&peer_fake_node)
-static struct inet_peer *peer_root = peer_avl_empty;
-static DEFINE_RWLOCK(peer_pool_lock);
+
+static struct {
+	struct inet_peer *root;
+	rwlock_t	 lock;
+	int		 total;
+} peers = {
+	.root	=	peer_avl_empty,
+	.lock	=	__RW_LOCK_UNLOCKED(peers.lock),
+};
 #define PEER_MAXDEPTH 40 /* sufficient for about 227 nodes */
 
-static int peer_total;
 /* Exported for sysctl_net_ipv4.  */
 int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries more
 					 * aggressively at this stage */
@@ -89,8 +95,13 @@  int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;	/* usual time to live: 10 min
 int inet_peer_gc_mintime __read_mostly = 10 * HZ;
 int inet_peer_gc_maxtime __read_mostly = 120 * HZ;
 
-static LIST_HEAD(unused_peers);
-static DEFINE_SPINLOCK(inet_peer_unused_lock);
+static struct {
+	struct list_head list;
+	spinlock_t	 lock;
+} unused_peers = {
+	.list	=	LIST_HEAD_INIT(unused_peers.list),
+	.lock	=	__SPIN_LOCK_UNLOCKED(unused_peers.lock),
+};
 
 static void peer_check_expire(unsigned long dummy);
 static DEFINE_TIMER(peer_periodic_timer, peer_check_expire, 0, 0);
@@ -131,9 +142,9 @@  void __init inet_initpeers(void)
 /* Called with or without local BH being disabled. */
 static void unlink_from_unused(struct inet_peer *p)
 {
-	spin_lock_bh(&inet_peer_unused_lock);
+	spin_lock_bh(&unused_peers.lock);
 	list_del_init(&p->unused);
-	spin_unlock_bh(&inet_peer_unused_lock);
+	spin_unlock_bh(&unused_peers.lock);
 }
 
 /*
@@ -146,9 +157,9 @@  static void unlink_from_unused(struct inet_peer *p)
 	struct inet_peer *u, **v;				\
 	if (_stack != NULL) {					\
 		stackptr = _stack;				\
-		*stackptr++ = &peer_root;			\
+		*stackptr++ = &peers.root;			\
 	}							\
-	for (u = peer_root; u != peer_avl_empty; ) {		\
+	for (u = peers.root; u != peer_avl_empty; ) {		\
 		if (_daddr == u->v4daddr)			\
 			break;					\
 		if ((__force __u32)_daddr < (__force __u32)u->v4daddr)	\
@@ -262,7 +273,7 @@  do {								\
 	n->avl_right = peer_avl_empty;				\
 	**--stackptr = n;					\
 	peer_avl_rebalance(stack, stackptr);			\
-} while(0)
+} while (0)
 
 /* May be called with local BH enabled. */
 static void unlink_from_pool(struct inet_peer *p)
@@ -271,7 +282,7 @@  static void unlink_from_pool(struct inet_peer *p)
 
 	do_free = 0;
 
-	write_lock_bh(&peer_pool_lock);
+	write_lock_bh(&peers.lock);
 	/* Check the reference counter.  It was artificially incremented by 1
 	 * in cleanup() function to prevent sudden disappearing.  If the
 	 * reference count is still 1 then the node is referenced only as `p'
@@ -303,10 +314,10 @@  static void unlink_from_pool(struct inet_peer *p)
 			delp[1] = &t->avl_left; /* was &p->avl_left */
 		}
 		peer_avl_rebalance(stack, stackptr);
-		peer_total--;
+		peers.total--;
 		do_free = 1;
 	}
-	write_unlock_bh(&peer_pool_lock);
+	write_unlock_bh(&peers.lock);
 
 	if (do_free)
 		kmem_cache_free(peer_cachep, p);
@@ -326,16 +337,16 @@  static int cleanup_once(unsigned long ttl)
 	struct inet_peer *p = NULL;
 
 	/* Remove the first entry from the list of unused nodes. */
-	spin_lock_bh(&inet_peer_unused_lock);
-	if (!list_empty(&unused_peers)) {
+	spin_lock_bh(&unused_peers.lock);
+	if (!list_empty(&unused_peers.list)) {
 		__u32 delta;
 
-		p = list_first_entry(&unused_peers, struct inet_peer, unused);
+		p = list_first_entry(&unused_peers.list, struct inet_peer, unused);
 		delta = (__u32)jiffies - p->dtime;
 
 		if (delta < ttl) {
 			/* Do not prune fresh entries. */
-			spin_unlock_bh(&inet_peer_unused_lock);
+			spin_unlock_bh(&unused_peers.lock);
 			return -1;
 		}
 
@@ -345,7 +356,7 @@  static int cleanup_once(unsigned long ttl)
 		 * before unlink_from_pool() call. */
 		atomic_inc(&p->refcnt);
 	}
-	spin_unlock_bh(&inet_peer_unused_lock);
+	spin_unlock_bh(&unused_peers.lock);
 
 	if (p == NULL)
 		/* It means that the total number of USED entries has
@@ -364,11 +375,11 @@  struct inet_peer *inet_getpeer(__be32 daddr, int create)
 	struct inet_peer **stack[PEER_MAXDEPTH], ***stackptr;
 
 	/* Look up for the address quickly. */
-	read_lock_bh(&peer_pool_lock);
+	read_lock_bh(&peers.lock);
 	p = lookup(daddr, NULL);
 	if (p != peer_avl_empty)
 		atomic_inc(&p->refcnt);
-	read_unlock_bh(&peer_pool_lock);
+	read_unlock_bh(&peers.lock);
 
 	if (p != peer_avl_empty) {
 		/* The existing node has been found. */
@@ -390,7 +401,7 @@  struct inet_peer *inet_getpeer(__be32 daddr, int create)
 	atomic_set(&n->ip_id_count, secure_ip_id(daddr));
 	n->tcp_ts_stamp = 0;
 
-	write_lock_bh(&peer_pool_lock);
+	write_lock_bh(&peers.lock);
 	/* Check if an entry has suddenly appeared. */
 	p = lookup(daddr, stack);
 	if (p != peer_avl_empty)
@@ -399,10 +410,10 @@  struct inet_peer *inet_getpeer(__be32 daddr, int create)
 	/* Link the node. */
 	link_to_pool(n);
 	INIT_LIST_HEAD(&n->unused);
-	peer_total++;
-	write_unlock_bh(&peer_pool_lock);
+	peers.total++;
+	write_unlock_bh(&peers.lock);
 
-	if (peer_total >= inet_peer_threshold)
+	if (peers.total >= inet_peer_threshold)
 		/* Remove one less-recently-used entry. */
 		cleanup_once(0);
 
@@ -411,7 +422,7 @@  struct inet_peer *inet_getpeer(__be32 daddr, int create)
 out_free:
 	/* The appropriate node is already in the pool. */
 	atomic_inc(&p->refcnt);
-	write_unlock_bh(&peer_pool_lock);
+	write_unlock_bh(&peers.lock);
 	/* Remove the entry from unused list if it was there. */
 	unlink_from_unused(p);
 	/* Free preallocated the preallocated node. */
@@ -425,12 +436,12 @@  static void peer_check_expire(unsigned long dummy)
 	unsigned long now = jiffies;
 	int ttl;
 
-	if (peer_total >= inet_peer_threshold)
+	if (peers.total >= inet_peer_threshold)
 		ttl = inet_peer_minttl;
 	else
 		ttl = inet_peer_maxttl
 				- (inet_peer_maxttl - inet_peer_minttl) / HZ *
-					peer_total / inet_peer_threshold * HZ;
+					peers.total / inet_peer_threshold * HZ;
 	while (!cleanup_once(ttl)) {
 		if (jiffies != now)
 			break;
@@ -439,22 +450,23 @@  static void peer_check_expire(unsigned long dummy)
 	/* Trigger the timer after inet_peer_gc_mintime .. inet_peer_gc_maxtime
 	 * interval depending on the total number of entries (more entries,
 	 * less interval). */
-	if (peer_total >= inet_peer_threshold)
+	if (peers.total >= inet_peer_threshold)
 		peer_periodic_timer.expires = jiffies + inet_peer_gc_mintime;
 	else
 		peer_periodic_timer.expires = jiffies
 			+ inet_peer_gc_maxtime
 			- (inet_peer_gc_maxtime - inet_peer_gc_mintime) / HZ *
-				peer_total / inet_peer_threshold * HZ;
+				peers.total / inet_peer_threshold * HZ;
 	add_timer(&peer_periodic_timer);
 }
 
 void inet_putpeer(struct inet_peer *p)
 {
-	spin_lock_bh(&inet_peer_unused_lock);
-	if (atomic_dec_and_test(&p->refcnt)) {
-		list_add_tail(&p->unused, &unused_peers);
+	local_bh_disable();
+	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
+		list_add_tail(&p->unused, &unused_peers.list);
 		p->dtime = (__u32)jiffies;
+		spin_unlock(&unused_peers.lock);
 	}
-	spin_unlock_bh(&inet_peer_unused_lock);
+	local_bh_enable();
 }