[PATCH v7 5/6] lib/dlock-list: Enable faster lookup with hashing

[Waiman Long <longman@redhat.com>]

Insertion and deletion is relatively cheap and mostly contention
free for dlock-list. Lookup, on the other hand, can be rather costly
because all the lists in a dlock-list will have to be iterated.

Currently dlock-list insertion is based on the cpu that the task is
running on. So a given object can be inserted into any one of the
lists depending on what the current cpu is.

This patch provides an alternative way of list selection. The caller
can provide a object context which will be hashed to one of the list
in a dlock-list. The object can then be added into that particular
list. Lookup can be done by iterating elements in the provided list
only instead of all the lists in a dlock-list.

The new APIs are:

struct dlock_list_head *dlock_list_hash(struct dlock_list_heads *, void *);
void dlock_list_add(struct dlock_list_node *, struct dlock_list_head *);

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/dlock-list.h |  9 +++++++++
 lib/dlock-list.c           | 49 +++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 51 insertions(+), 7 deletions(-)

diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
index 7940e524..16474ae 100644
--- a/include/linux/dlock-list.h
+++ b/include/linux/dlock-list.h
@@ -121,6 +121,15 @@ extern void dlock_lists_add(struct dlock_list_node *node,
 extern void dlock_lists_del(struct dlock_list_node *node);
 
 /*
+ * Instead of individual list mapping by CPU number, it can be based on
+ * a given context to speed up loockup performance.
+ */
+extern struct dlock_list_head *dlock_list_hash(struct dlock_list_heads *dlist,
+					       void *context);
+extern void dlock_list_add(struct dlock_list_node *node,
+			   struct dlock_list_head *head);
+
+/*
  * Find the first entry of the next available list.
  */
 extern struct dlock_list_node *
diff --git a/lib/dlock-list.c b/lib/dlock-list.c
index a045fd7..8cd0876 100644
--- a/lib/dlock-list.c
+++ b/lib/dlock-list.c
@@ -20,6 +20,7 @@
 #include <linux/lockdep.h>
 #include <linux/slab.h>
 #include <linux/cpumask.h>
+#include <linux/jhash.h>
 
 /*
  * The distributed and locked list is a distributed set of lists each of
@@ -163,6 +164,46 @@ bool dlock_lists_empty(struct dlock_list_heads *dlist)
 }
 
 /**
+ * dlock_list_hash - Hash the given context to a particular list
+ * @dlist: The dlock list
+ * @ctx  : The context for hashing
+ */
+struct dlock_list_head *dlock_list_hash(struct dlock_list_heads *dlist,
+					void *ctx)
+{
+	unsigned long val = (unsigned long)ctx;
+	u32 hash;
+
+	if (unlikely(!nr_dlock_lists)) {
+		WARN_ON_ONCE(1);
+		return &dlist->heads[0];
+	}
+	if (val < nr_dlock_lists)
+		hash = val;
+	else
+		hash = jhash2((u32 *)&ctx, sizeof(ctx)/sizeof(u32), 0)
+				% nr_dlock_lists;
+	return &dlist->heads[hash];
+}
+
+/**
+ * dlock_list_add - Add a node to a particular head of dlock list
+ * @node: The node to be added
+ * @head: The dlock list head where the node is to be added
+ */
+void dlock_list_add(struct dlock_list_node *node,
+		    struct dlock_list_head *head)
+{
+	/*
+	 * There is no need to disable preemption
+	 */
+	spin_lock(&head->lock);
+	node->head = head;
+	list_add(&node->list, &head->list);
+	spin_unlock(&head->lock);
+}
+
+/**
  * dlock_lists_add - Adds a node to the given dlock list
  * @node : The node to be added
  * @dlist: The dlock list where the node is to be added
@@ -175,13 +216,7 @@ void dlock_lists_add(struct dlock_list_node *node,
 {
 	struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)];
 
-	/*
-	 * There is no need to disable preemption
-	 */
-	spin_lock(&head->lock);
-	node->head = head;
-	list_add(&node->list, &head->list);
-	spin_unlock(&head->lock);
+	dlock_list_add(node, head);
 }
 
 /**
-- 
1.8.3.1