Re: [PATCH v8 1/6] lib/dlock-list: Distributed and lock-protected lists
From: Davidlohr Bueso
Date: Fri Nov 03 2017 - 09:35:29 EST
On Thu, 02 Nov 2017, Waiman Long wrote:
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().
The counter will then become the single contention point for all
concurrent updates to the dlock-list. So it will have a big impact on
performance. On the other hand, instead of being a counter of # of
items, we can make that a counter of # of non-empty lists. So its value
will only be changed when a list go from empty to non-empty and vice
versa. That will greatly reduce the number of updates to that counter.
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
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;
};
I don't want to add a new data item into dlock_list_node as there can be
thousands or even of them in the system. Instead, I prefer increasing the
size of dlock_list_head which only have a limited number of them and
they have unused space because they are cacheline aligned.
Both are good points. Thanks.
----8<--------------------------------------------------------
Subject: [PATCH] [PATCH v2] lib/dlock-list: Scale dlock_lists_empty()
Instead of the current O(N) implementation, at the cost
of adding an atomic counter, we can convert the call to
an atomic_read(). The counter only serves for accounting
empty to non-empty transitions, and vice versa; therefore
only modified twice for each of the lists, during the
lifetime of the dlock -- thus 2*nr_dlock_lists.
In addition, to be able to unaccount a list_del(), we
add a dlist pointer to each head, thus minimizing the
overall memory footprint.
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
include/linux/dlock-list.h | 2 ++
lib/dlock-list.c | 56 +++++++++++++++++++++++++++++++++++-----------
2 files changed, 45 insertions(+), 13 deletions(-)
diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
index c00c7f92ada4..d176a2d00cd1 100644
--- a/include/linux/dlock-list.h
+++ b/include/linux/dlock-list.h
@@ -32,10 +32,12 @@
struct dlock_list_head {
struct list_head list;
spinlock_t lock;
+ struct dlock_list_heads *dlist;
} ____cacheline_aligned_in_smp;
struct dlock_list_heads {
struct dlock_list_head *heads;
+ atomic_t waiters;
};
/*
diff --git a/lib/dlock-list.c b/lib/dlock-list.c
index a4ddecc01b12..a84f42e800d5 100644
--- a/lib/dlock-list.c
+++ b/lib/dlock-list.c
@@ -122,8 +122,11 @@ int __alloc_dlock_list_heads(struct dlock_list_heads *dlist,
INIT_LIST_HEAD(&head->list);
head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
+ head->dlist = dlist;
lockdep_set_class(&head->lock, key);
}
+
+ atomic_set(&dlist->waiters, 0);
return 0;
}
EXPORT_SYMBOL(__alloc_dlock_list_heads);
@@ -138,30 +141,36 @@ EXPORT_SYMBOL(__alloc_dlock_list_heads);
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;
+ /*
+ * 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()
+ *
+ */
+ smp_mb__before_atomic();
+ return !atomic_read(&dlist->waiters);
}
EXPORT_SYMBOL(dlock_lists_empty);
@@ -179,6 +188,16 @@ void dlock_lists_add(struct dlock_list_node *node,
struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)];
/*
+ * 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.
+ */
+ if (list_empty_careful(&head->list)) {
+ atomic_inc(&dlist->waiters);
+ smp_mb__after_atomic();
+ }
+
+ /*
* There is no need to disable preemption
*/
spin_lock(&head->lock);
@@ -199,8 +218,7 @@ 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 {
@@ -212,6 +230,18 @@ void dlock_lists_del(struct dlock_list_node *node)
spin_lock(&head->lock);
if (likely(head == node->head)) {
list_del_init(&node->list);
+ /*
+ * We still hold the head->lock, a normal list_empty()
+ * check will do.
+ */
+ if (list_empty(&head->list)) {
+ struct dlock_list_heads *dlist;
+ dlist = node->head->dlist;
+
+ atomic_dec(&dlist->waiters);
+ smp_mb__after_atomic();
+ }
+
node->head = NULL;
retry = false;
} else {
--
2.13.6