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, 16 insertions, 22 deletions

diff --git a/lib/idr.c b/lib/idr.c
index 5cd602936645..1a30272066c6 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -137,8 +137,10 @@ static void idr_layer_rcu_free(struct rcu_head *head)
         kmem_cache_free(idr_layer_cache, layer);
 }
 
-static inline void free_layer(struct idr_layer *p)
+static inline void free_layer(struct idr *idr, struct idr_layer *p)
 {
+        if (idr->hint && idr->hint == p)
+                RCU_INIT_POINTER(idr->hint, NULL);
         call_rcu(&p->rcu_head, idr_layer_rcu_free);
 }
 
@@ -363,8 +365,12 @@ build_up:
  * @id and @pa are from a successful allocation from idr_get_empty_slot().
  * Install the user pointer @ptr and mark the slot full.
  */
-static void idr_fill_slot(void *ptr, int id, struct idr_layer **pa)
+static void idr_fill_slot(struct idr *idr, void *ptr, int id,
+                          struct idr_layer **pa)
 {
+        /* update hint used for lookup, cleared from free_layer() */
+        rcu_assign_pointer(idr->hint, pa[0]);
+
         rcu_assign_pointer(pa[0]->ary[id & IDR_MASK], (struct idr_layer *)ptr);
         pa[0]->count++;
         idr_mark_full(pa, id);
@@ -397,7 +403,7 @@ int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id)
         if (rv < 0)
                 return rv == -ENOMEM ? -EAGAIN : rv;
 
-        idr_fill_slot(ptr, rv, pa);
+        idr_fill_slot(idp, ptr, rv, pa);
         *id = rv;
         return 0;
 }
@@ -504,7 +510,7 @@ int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp_mask)
         if (unlikely(id > max))
                 return -ENOSPC;
 
-        idr_fill_slot(ptr, id, pa);
+        idr_fill_slot(idr, ptr, id, pa);
         return id;
 }
 EXPORT_SYMBOL_GPL(idr_alloc);
@@ -541,14 +547,14 @@ static void sub_remove(struct idr *idp, int shift, int id)
                 to_free = NULL;
                 while(*paa && ! --((**paa)->count)){
                         if (to_free)
-                                free_layer(to_free);
+                                free_layer(idp, to_free);
                         to_free = **paa;
                         **paa-- = NULL;
                 }
                 if (!*paa)
                         idp->layers = 0;
                 if (to_free)
-                        free_layer(to_free);
+                        free_layer(idp, to_free);
         } else
                 idr_remove_warning(id);
 }
@@ -581,7 +587,7 @@ void idr_remove(struct idr *idp, int id)
                 --idp->layers;
                 to_free->count = 0;
                 bitmap_clear(to_free->bitmap, 0, IDR_SIZE);
-                free_layer(to_free);
+                free_layer(idp, to_free);
         }
         while (idp->id_free_cnt >= MAX_IDR_FREE) {
                 p = get_from_free_list(idp);
@@ -622,7 +628,7 @@ void __idr_remove_all(struct idr *idp)
                 /* Get the highest bit that the above add changed from 0->1. */
                 while (n < fls(id ^ bt_mask)) {
                         if (p)
-                                free_layer(p);
+                                free_layer(idp, p);
                         n += IDR_BITS;
                         p = *--paa;
                 }
@@ -655,19 +661,7 @@ void idr_destroy(struct idr *idp)
 }
 EXPORT_SYMBOL(idr_destroy);
 
-/**
- * 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.
- */
-void *idr_find(struct idr *idp, int id)
+void *idr_find_slowpath(struct idr *idp, int id)
 {
         int n;
         struct idr_layer *p;
@@ -691,7 +685,7 @@ void *idr_find(struct idr *idp, int id)
         }
         return((void *)p);
 }
-EXPORT_SYMBOL(idr_find);
+EXPORT_SYMBOL(idr_find_slowpath);
 
 /**
  * idr_for_each - iterate through all stored pointers