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Date:   Thu, 2 Nov 2017 10:04:31 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v8 1/6] lib/dlock-list: Distributed and lock-protected
 lists

On Tue, 31 Oct 2017, Waiman Long wrote:

>+/**
>+ * dlock_lists_empty - Check if all the dlock lists are empty
>+ * @dlist: Pointer to the dlock_list_heads structure
>+ * Return: true if list is empty, false otherwise.
>+ *           
>+ * This can be a pretty expensive function call. If this function is required
>+ * in a performance critical path, we may have to maintain a global count
>+ * of the list entries in the global dlock_list_heads structure instead.
>+ */

I vote for doing this in the original version. How about the following?

>+bool dlock_lists_empty(struct dlock_list_heads *dlist)
>+{
>+	int idx;
>+
>+	for (idx = 0; idx < nr_cpu_ids; idx++)
>+		if (!list_empty(&dlist->heads[idx].list))
>+			return false;
>+	return true;
>+}
>+EXPORT_SYMBOL(dlock_lists_empty);

----------8<-----------------------------------------------
From: Davidlohr Bueso <dave@...olabs.net>
Subject: [PATCH] lib/dlock-list: Scale dlock_lists_empty()

Instead of the current O(N) implementation; at the cost
of adding an atomic counter. We also need to add a heads
pointer to the node structure such that we can unaccount
a thread doing list_del().

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 include/linux/dlock-list.h |  2 ++
 lib/dlock-list.c           | 40 ++++++++++++++++++++++++++++------------
 2 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
index c00c7f92ada4..dd73d5787885 100644
--- a/include/linux/dlock-list.h
+++ b/include/linux/dlock-list.h
@@ -36,6 +36,7 @@ struct dlock_list_head {
 
 struct dlock_list_heads {
 	struct dlock_list_head *heads;
+	atomic_t waiters;
 };
 
 /*
@@ -44,6 +45,7 @@ struct dlock_list_heads {
 struct dlock_list_node {
 	struct list_head list;
 	struct dlock_list_head *head;
+	struct dlock_list_heads *heads;
 };
 
 /*
diff --git a/lib/dlock-list.c b/lib/dlock-list.c
index a4ddecc01b12..bd11fc0da254 100644
--- a/lib/dlock-list.c
+++ b/lib/dlock-list.c
@@ -124,6 +124,8 @@ int __alloc_dlock_list_heads(struct dlock_list_heads *dlist,
 		head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
 		lockdep_set_class(&head->lock, key);
 	}
+
+	atomic_set(&dlist->waiters, 0);
 	return 0;
 }
 EXPORT_SYMBOL(__alloc_dlock_list_heads);
@@ -139,29 +141,23 @@ void free_dlock_list_heads(struct dlock_list_heads *dlist)
 {
 	kfree(dlist->heads);
 	dlist->heads = NULL;
+	atomic_set(&dlist->waiters, 0);
 }
 EXPORT_SYMBOL(free_dlock_list_heads);
 
 /**
  * dlock_lists_empty - Check if all the dlock lists are empty
  * @dlist: Pointer to the dlock_list_heads structure
- * Return: true if list is empty, false otherwise.
  *
- * This can be a pretty expensive function call. If this function is required
- * in a performance critical path, we may have to maintain a global count
- * of the list entries in the global dlock_list_heads structure instead.
+ * Return: true if all dlock lists are empty, false otherwise.
  */
 bool dlock_lists_empty(struct dlock_list_heads *dlist)
 {
-	int idx;
-
 	/* Shouldn't be called before nr_dlock_lists is initialized */
 	WARN_ON_ONCE(!nr_dlock_lists);
 
-	for (idx = 0; idx < nr_dlock_lists; idx++)
-		if (!list_empty(&dlist->heads[idx].list))
-			return false;
-	return true;
+	smp_mb__before_atomic();
+	return !atomic_read(&dlist->waiters);
 }
 EXPORT_SYMBOL(dlock_lists_empty);
 
@@ -179,10 +175,30 @@ void dlock_lists_add(struct dlock_list_node *node,
 	struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)];
 
 	/*
+	 * Serialize dlist->waiters such that a 0->1 transition is not missed,
+	 * by another thread checking if any of the dlock lists are used.
+	 *
+	 * CPU0				    CPU1
+	 * dlock_list_add()                 dlock_lists_empty()
+	 *   [S] atomic_inc(waiters);
+	 *	 smp_mb__after_atomic();
+	 *				      smp_mb__before_atomic();
+	 *				      [L] atomic_read(waiters)
+	 *       list_add()
+	 *
+	 * Bump the waiters counter _before_ taking the head->lock such that we
+	 * don't miss a thread adding itself to a list while spinning for the
+	 * lock.
+	 */
+	atomic_inc(&dlist->waiters);
+	smp_mb__after_atomic();
+
+	/*
 	 * There is no need to disable preemption
 	 */
 	spin_lock(&head->lock);
 	node->head = head;
+	node->heads = dlist;
 	list_add(&node->list, &head->list);
 	spin_unlock(&head->lock);
 }
@@ -199,8 +215,7 @@ EXPORT_SYMBOL(dlock_lists_add);
  * a bug.
  */
 void dlock_lists_del(struct dlock_list_node *node)
-{
-	struct dlock_list_head *head;
+{	struct dlock_list_head *head;
 	bool retry;
 
 	do {
@@ -214,6 +229,7 @@ void dlock_lists_del(struct dlock_list_node *node)
 			list_del_init(&node->list);
 			node->head = NULL;
 			retry = false;
+			atomic_dec(&node->heads->waiters);
 		} else {
 			/*
 			 * The lock has somehow changed. Retry again if it is
-- 
2.13.6

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