idr: implement lookup hint

While idr lookup isn't a particularly heavy operation, it still is too
substantial to use in hot paths without worrying about the performance
implications.  With recent changes, each idr_layer covers 256 slots
which should be enough to cover most use cases with single idr_layer
making lookup hint very attractive.

This patch adds idr->hint which points to the idr_layer which
allocated an ID most recently and the fast path lookup becomes

	if (look up target's prefix matches that of the hinted layer)
		return hint->ary[ID's offset in the leaf layer];

which can be inlined.

idr->hint is set to the leaf node on idr_fill_slot() and cleared from
free_layer().

[andriy.shevchenko@linux.intel.com: always do slow path when hint is uninitialized]
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 24 insertions, 1 deletions

diff --git a/include/linux/idr.h b/include/linux/idr.h
index 7b1c5c6f9a0..a6f38b5c34e 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -37,6 +37,7 @@ struct idr_layer {
 };
 
 struct idr {
+	struct idr_layer __rcu	*hint;	/* the last layer allocated from */
 	struct idr_layer __rcu	*top;
 	struct idr_layer	*id_free;
 	int			layers;	/* only valid w/o concurrent changes */
@@ -71,7 +72,7 @@ struct idr {
  * This is what we export.
  */
 
-void *idr_find(struct idr *idp, int id);
+void *idr_find_slowpath(struct idr *idp, int id);
 int idr_pre_get(struct idr *idp, gfp_t gfp_mask);
 int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id);
 void idr_preload(gfp_t gfp_mask);
@@ -97,6 +98,28 @@ static inline void idr_preload_end(void)
 }
 
 /**
+ * idr_find - return pointer for given id
+ * @idp: idr handle
+ * @id: lookup key
+ *
+ * Return the pointer given the id it has been registered with.  A %NULL
+ * return indicates that @id is not valid or you passed %NULL in
+ * idr_get_new().
+ *
+ * This function can be called under rcu_read_lock(), given that the leaf
+ * pointers lifetimes are correctly managed.
+ */
+static inline void *idr_find(struct idr *idr, int id)
+{
+	struct idr_layer *hint = rcu_dereference_raw(idr->hint);
+
+	if (hint && (id & ~IDR_MASK) == hint->prefix)
+		return rcu_dereference_raw(hint->ary[id & IDR_MASK]);
+
+	return idr_find_slowpath(idr, id);
+}
+
+/**
  * idr_get_new - allocate new idr entry
  * @idp: idr handle
  * @ptr: pointer you want associated with the id