[PATCH 15/17] powerpc/qspinlock: reduce remote node steal spins

[Nicholas Piggin]

Allow for a reduction in the number of times a CPU from a different
node than the owner can attempt to steal the lock before queueing.
This could bias the transfer behaviour of the lock across the
machine and reduce NUMA crossings.
---
 arch/powerpc/lib/qspinlock.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index d4594c701f7d..24f68bd71e2b 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -4,6 +4,7 @@
 #include <linux/export.h>
 #include <linux/percpu.h>
 #include <linux/smp.h>
+#include <linux/topology.h>
 #include <asm/qspinlock.h>
 #include <asm/paravirt.h>
 
@@ -24,6 +25,7 @@ struct qnodes {
 
 /* Tuning parameters */
 static int STEAL_SPINS __read_mostly = (1<<5);
+static int REMOTE_STEAL_SPINS __read_mostly = (1<<2);
 #if _Q_SPIN_TRY_LOCK_STEAL == 1
 static const bool MAYBE_STEALERS = true;
 #else
@@ -39,9 +41,13 @@ static bool pv_prod_head __read_mostly = false;
 
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 
-static __always_inline int get_steal_spins(bool paravirt)
+static __always_inline int get_steal_spins(bool paravirt, bool remote)
 {
-	return STEAL_SPINS;
+	if (remote) {
+		return REMOTE_STEAL_SPINS;
+	} else {
+		return STEAL_SPINS;
+	}
 }
 
 static __always_inline int get_head_spins(bool paravirt)
@@ -380,8 +386,13 @@ static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool parav
 
 		iters++;
 
-		if (iters >= get_steal_spins(paravirt))
+		if (iters >= get_steal_spins(paravirt, false))
 			break;
+		if (iters >= get_steal_spins(paravirt, true)) {
+			int cpu = get_owner_cpu(val);
+			if (numa_node_id() != cpu_to_node(cpu))
+				break;
+		}
 	}
 	spin_end();
 
@@ -588,6 +599,22 @@ static int steal_spins_get(void *data, u64 *val)
 
 DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_spins_set, "%llu\n");
 
+static int remote_steal_spins_set(void *data, u64 val)
+{
+	REMOTE_STEAL_SPINS = val;
+
+	return 0;
+}
+
+static int remote_steal_spins_get(void *data, u64 *val)
+{
+	*val = REMOTE_STEAL_SPINS;
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_remote_steal_spins, remote_steal_spins_get, remote_steal_spins_set, "%llu\n");
+
 static int head_spins_set(void *data, u64 val)
 {
 	HEAD_SPINS = val;
@@ -687,6 +714,7 @@ DEFINE_SIMPLE_ATTRIBUTE(fops_pv_prod_head, pv_prod_head_get, pv_prod_head_set, "
 static __init int spinlock_debugfs_init(void)
 {
 	debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_steal_spins);
+	debugfs_create_file("qspl_remote_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_remote_steal_spins);
 	debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL, &fops_head_spins);
 	if (is_shared_processor()) {
 		debugfs_create_file("qspl_pv_yield_owner", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_owner);
-- 
2.35.1