[v5,4/5] locking/qspinlock: Introduce starvation avoidance into CNA
diff mbox series

Message ID 20191016042903.61081-5-alex.kogan@oracle.com
State New
Headers show
Series
  • Add NUMA-awareness to qspinlock
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Commit Message

Alex Kogan Oct. 16, 2019, 4:29 a.m. UTC
Keep track of the number of intra-node lock handoffs, and force
inter-node handoff once this number reaches a preset threshold.

Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
---
 kernel/locking/qspinlock.c     |  3 +++
 kernel/locking/qspinlock_cna.h | 30 +++++++++++++++++++++++++++---
 2 files changed, 30 insertions(+), 3 deletions(-)

Comments

Waiman Long Oct. 18, 2019, 4:09 p.m. UTC | #1
On 10/16/19 12:29 AM, Alex Kogan wrote:
> Keep track of the number of intra-node lock handoffs, and force
> inter-node handoff once this number reaches a preset threshold.
>
> Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
> Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>  kernel/locking/qspinlock.c     |  3 +++
>  kernel/locking/qspinlock_cna.h | 30 +++++++++++++++++++++++++++---
>  2 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 6d8c4a52e44e..1d0d884308ef 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -597,6 +597,9 @@ EXPORT_SYMBOL(queued_spin_lock_slowpath);
>  #if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>  #define _GEN_CNA_LOCK_SLOWPATH
>  
> +#undef pv_init_node
> +#define pv_init_node			cna_init_node
> +
>  #undef pv_wait_head_or_lock
>  #define pv_wait_head_or_lock		cna_pre_scan
>  
> diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
> index 4d095f742d31..b92a6f9a19db 100644
> --- a/kernel/locking/qspinlock_cna.h
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -50,9 +50,19 @@ struct cna_node {
>  	struct mcs_spinlock	mcs;
>  	int			numa_node;
>  	u32			encoded_tail;
> -	u32			pre_scan_result; /* 0 or an encoded tail */
> +	u32			pre_scan_result; /* 0, 1 or an encoded tail */
> +	u32			intra_count;
>  };
>  
> +/*
> + * Controls the threshold for the number of intra-node lock hand-offs. It can
> + * be tuned and depend, e.g., on the number of CPUs per node. For now,
> + * choose a value that provides reasonable long-term fairness without
> + * sacrificing performance compared to a version that does not have any
> + * fairness guarantees.
> + */
> +#define INTRA_NODE_HANDOFF_THRESHOLD (1 << 16)

I think 64k is too high. I will be more comfortable with a number like
(1 << 8). The worst case latency for a lock waiter from the other node
is just not acceptable.

Cheers,
Longman

Patch
diff mbox series

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 6d8c4a52e44e..1d0d884308ef 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -597,6 +597,9 @@  EXPORT_SYMBOL(queued_spin_lock_slowpath);
 #if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
 #define _GEN_CNA_LOCK_SLOWPATH
 
+#undef pv_init_node
+#define pv_init_node			cna_init_node
+
 #undef pv_wait_head_or_lock
 #define pv_wait_head_or_lock		cna_pre_scan
 
diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
index 4d095f742d31..b92a6f9a19db 100644
--- a/kernel/locking/qspinlock_cna.h
+++ b/kernel/locking/qspinlock_cna.h
@@ -50,9 +50,19 @@  struct cna_node {
 	struct mcs_spinlock	mcs;
 	int			numa_node;
 	u32			encoded_tail;
-	u32			pre_scan_result; /* 0 or an encoded tail */
+	u32			pre_scan_result; /* 0, 1 or an encoded tail */
+	u32			intra_count;
 };
 
+/*
+ * Controls the threshold for the number of intra-node lock hand-offs. It can
+ * be tuned and depend, e.g., on the number of CPUs per node. For now,
+ * choose a value that provides reasonable long-term fairness without
+ * sacrificing performance compared to a version that does not have any
+ * fairness guarantees.
+ */
+#define INTRA_NODE_HANDOFF_THRESHOLD (1 << 16)
+
 static void __init cna_init_nodes_per_cpu(unsigned int cpu)
 {
 	struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
@@ -86,6 +96,11 @@  static void __init cna_init_nodes(void)
 }
 early_initcall(cna_init_nodes);
 
+static __always_inline void cna_init_node(struct mcs_spinlock *node)
+{
+	((struct cna_node *)node)->intra_count = 0;
+}
+
 static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val,
 				       struct mcs_spinlock *node)
 {
@@ -215,7 +230,13 @@  __always_inline u32 cna_pre_scan(struct qspinlock *lock,
 {
 	struct cna_node *cn = (struct cna_node *)node;
 
-	cn->pre_scan_result = cna_scan_main_queue(node, node);
+	/*
+	 * setting @pre_scan_result to 1 indicates that no post-scan
+	 * should be made in cna_pass_lock()
+	 */
+	cn->pre_scan_result =
+		cn->intra_count == INTRA_NODE_HANDOFF_THRESHOLD ?
+			1 : cna_scan_main_queue(node, node);
 
 	return 0;
 }
@@ -234,7 +255,7 @@  static inline void cna_pass_lock(struct mcs_spinlock *node,
 	 * pre-scan, and if so, try to find it in post-scan starting from the
 	 * node where pre-scan stopped (stored in @pre_scan_result)
 	 */
-	if (scan > 0)
+	if (scan > 1)
 		scan = cna_scan_main_queue(node, decode_tail(scan));
 
 	if (!scan) { /* if found a successor from the same numa node */
@@ -245,6 +266,9 @@  static inline void cna_pass_lock(struct mcs_spinlock *node,
 		 * to pass the lock
 		 */
 		val = node->locked + (node->locked == 0);
+		/* inc @intra_count if the secondary queue is not empty */
+		((struct cna_node *)next_holder)->intra_count =
+			cn->intra_count + (node->locked > 1);
 	} else if (node->locked > 1) {	  /* if secondary queue is not empty */
 		/* next holder will be the first node in the secondary queue */
 		tail_2nd = decode_tail(node->locked);