sysvipc: make get_maxid O(1) again

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 commit 7ca7e56 ("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. [[email protected]: coding-style fixes] Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Davidlohr Bueso <[email protected]> Cc: Manfred Spraul <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
wafgo · Nov 18, 2017 · 15df03c · 15df03c
1 parent ebf6679
commit 15df03c
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Showing 3 changed files with 32 additions and 33 deletions.
diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
@@ -19,6 +19,7 @@ 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
@@ -122,6 +122,7 @@ 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
@@ -188,36 +189,6 @@ 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.
@@ -313,6 +284,9 @@ 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;
@@ -459,6 +433,15 @@ 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
@@ -13,6 +13,7 @@
 
 #include <linux/unistd.h>
 #include <linux/err.h>
+#include <linux/ipc_namespace.h>
 
 #define SEQ_MULTIPLIER	(IPCMNI)
 
@@ -99,9 +100,6 @@ 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 *);
 
@@ -111,6 +109,23 @@ 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.