[PATCH v2 4/4] sysvipc: make get_maxid O(1) again

[Davidlohr Bueso]

For a custom microbenchmark on a 3.30GHz Xeon SandyBridge,
which calls IPC_STAT over and over, it was calculated that,
on avg the cost of ipc_get_maxid() for increasing amounts of
keys was:

10 keys: ~900 cycles
100 keys: ~15000 cycles
1000 keys: ~150000 cycles
10000 keys: ~2100000 cycles

This is unsurprising as maxid is currently O(n).

By having the max_id available in O(1) we save all those cycles
for each semctl(_STAT) command, the idr_find can be expensive --
which some real (customer) workloads actually poll on. Note that
this used to be the case, until 7ca7e564e04 (ipc: store ipcs into
IDRs). The cost is the extra idr_find when doing RMIDs, but we
simply go backwards, and should not take too many iterations to
find the new value.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
 include/linux/ipc_namespace.h |  1 +
 ipc/util.c                    | 43 +++++++++++++------------------------------
 ipc/util.h                    | 21 ++++++++++++++++++---
 3 files changed, 32 insertions(+), 33 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 058051566ced..c59b8ddee191 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -18,6 +18,7 @@ struct ipc_ids {
 	bool tables_initialized;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
+	int max_id;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	int next_id;
 #endif
diff --git a/ipc/util.c b/ipc/util.c
index d4091665f439..45e84203c3fe 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -121,6 +121,7 @@ int ipc_init_ids(struct ipc_ids *ids)
 		return err;
 	idr_init(&ids->ipcs_idr);
 	ids->tables_initialized = true;
+	ids->max_id = -1;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	ids->next_id = -1;
 #endif
@@ -187,36 +188,6 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
 	return NULL;
 }
 
-/**
- * ipc_get_maxid - get the last assigned id
- * @ids: ipc identifier set
- *
- * Called with ipc_ids.rwsem held.
- */
-int ipc_get_maxid(struct ipc_ids *ids)
-{
-	struct kern_ipc_perm *ipc;
-	int max_id = -1;
-	int total, id;
-
-	if (ids->in_use == 0)
-		return -1;
-
-	if (ids->in_use == IPCMNI)
-		return IPCMNI - 1;
-
-	/* Look for the last assigned id */
-	total = 0;
-	for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL) {
-			max_id = id;
-			total++;
-		}
-	}
-	return max_id;
-}
-
 #ifdef CONFIG_CHECKPOINT_RESTORE
 /*
  * Specify desired id for next allocated IPC object.
@@ -312,6 +283,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	}
 
 	ids->in_use++;
+	if (id > ids->max_id)
+		ids->max_id = id;
+
 	new->id = ipc_buildid(id, ids, new);
 
 	return id;
@@ -458,6 +432,15 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 	ipc_kht_remove(ids, ipcp);
 	ids->in_use--;
 	ipcp->deleted = true;
+
+	if (unlikely(lid == ids->max_id)) {
+		do {
+			lid--;
+			if(lid == -1)
+				break;
+		} while (!idr_find(&ids->ipcs_idr, lid));
+		ids->max_id = lid;
+	}
 }
 
 /**
diff --git a/ipc/util.h b/ipc/util.h
index 17c8d2f14990..eabca0e37ce1 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -12,6 +12,7 @@
 
 #include <linux/unistd.h>
 #include <linux/err.h>
+#include <linux/ipc_namespace.h>
 
 #define SEQ_MULTIPLIER	(IPCMNI)
 
@@ -98,9 +99,6 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
 /* must be called with ids->rwsem acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
 
-/* must be called with ids->rwsem acquired for reading */
-int ipc_get_maxid(struct ipc_ids *);
-
 /* must be called with both locks acquired. */
 void ipc_rmid(struct ipc_ids *, struct kern_ipc_perm *);
 
@@ -110,6 +108,23 @@ void ipc_set_key_private(struct ipc_ids *, struct kern_ipc_perm *);
 /* must be called with ipcp locked */
 int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
 
+/**
+ * ipc_get_maxid - get the last assigned id
+ * @ids: ipc identifier set
+ *
+ * Called with ipc_ids.rwsem held for reading.
+ */
+static inline int ipc_get_maxid(struct ipc_ids *ids)
+{
+	if (ids->in_use == 0)
+		return -1;
+
+	if (ids->in_use == IPCMNI)
+		return IPCMNI - 1;
+
+	return ids->max_id;
+}
+
 /*
  * For allocation that need to be freed by RCU.
  * Objects are reference counted, they start with reference count 1.
-- 
2.12.0