Subject: [PATCH 11/17] powerpc/qspinlock: allow propagation of yield CPU down the queue

From: Nicholas Piggin <[email-masked]>
Date: Thu, 28 Jul 2022 16:31:14 +1000

Having all CPUs poll the lock word for the owner CPU that should be
yielded to defeats most of the purpose of using MCS queueing for
scalability. Yet it may be desirable for queued waiters to to yield
to a preempted owner.

s390 addreses this problem by having queued waiters sample the lock
word to find the owner much less frequently. In this approach, the
waiters never sample it directly, but the queue head propagates the
owner CPU back to the next waiter if it ever finds the owner has
been preempted. Queued waiters then subsequently propagate the owner
CPU back to the next waiter, and so on.

Disable this option by default for now, i.e., no logical change.
---
 arch/powerpc/lib/qspinlock.c | 85 +++++++++++++++++++++++++++++++++++-
 1 file changed, 84 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 94f007f66942..28c85a2d5635 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -12,6 +12,7 @@
 struct qnode {
 	struct qnode	*next;
 	struct qspinlock *lock;
+	int		yield_cpu;
 	u8		locked; /* 1 if lock acquired */
 };
 
@@ -28,6 +29,7 @@ static int HEAD_SPINS __read_mostly = (1<<8);
 static bool pv_yield_owner __read_mostly = true;
 static bool pv_yield_allow_steal __read_mostly = false;
 static bool pv_yield_prev __read_mostly = true;
+static bool pv_yield_propagate_owner __read_mostly = true;
 
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 
@@ -257,13 +259,66 @@ static __always_inline void yield_head_to_locked_owner(struct qspinlock *lock, u
 	__yield_to_locked_owner(lock, val, paravirt, clear_mustq);
 }
 
+static __always_inline void propagate_yield_cpu(struct qnode *node, u32 val, int *set_yield_cpu, bool paravirt)
+{
+	struct qnode *next;
+	int owner;
+
+	if (!paravirt)
+		return;
+	if (!pv_yield_propagate_owner)
+		return;
+
+	owner = get_owner_cpu(val);
+	if (*set_yield_cpu == owner)
+		return;
+
+	next = READ_ONCE(node->next);
+	if (!next)
+		return;
+
+	if (vcpu_is_preempted(owner)) {
+		next->yield_cpu = owner;
+		*set_yield_cpu = owner;
+	} else if (*set_yield_cpu != -1) {
+		next->yield_cpu = owner;
+		*set_yield_cpu = owner;
+	}
+}
+
 static __always_inline void yield_to_prev(struct qspinlock *lock, struct qnode *node, int prev_cpu, bool paravirt)
 {
 	u32 yield_count;
+	int yield_cpu;
 
 	if (!paravirt)
 		goto relax;
 
+	if (!pv_yield_propagate_owner)
+		goto yield_prev;
+
+	yield_cpu = READ_ONCE(node->yield_cpu);
+	if (yield_cpu == -1) {
+		/* Propagate back the -1 CPU */
+		if (node->next && node->next->yield_cpu != -1)
+			node->next->yield_cpu = yield_cpu;
+		goto yield_prev;
+	}
+
+	yield_count = yield_count_of(yield_cpu);
+	if ((yield_count & 1) == 0)
+		goto yield_prev; /* owner vcpu is running */
+
+	smp_rmb();
+
+	if (yield_cpu == node->yield_cpu) {
+		if (node->next && node->next->yield_cpu != yield_cpu)
+			node->next->yield_cpu = yield_cpu;
+		yield_to_preempted(yield_cpu, yield_count);
+		return;
+	}
+
+yield_prev:
 	if (!pv_yield_prev)
 		goto relax;
 
@@ -337,6 +392,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
 	node = &qnodesp->nodes[idx];
 	node->next = NULL;
 	node->lock = lock;
+	node->yield_cpu = -1;
 	node->locked = 0;
 
 	tail = encode_tail_cpu();
@@ -358,13 +414,21 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
 		while (!node->locked)
 			yield_to_prev(lock, node, prev_cpu, paravirt);
 
+		/* Clear out stale propagated yield_cpu */
+		if (paravirt && pv_yield_propagate_owner && node->yield_cpu != -1)
+			node->yield_cpu = -1;
+
 		smp_rmb(); /* acquire barrier for the mcs lock */
 	}
 
 	if (!MAYBE_STEALERS) {
+		int set_yield_cpu = -1;
+
 		/* We're at the head of the waitqueue, wait for the lock. */
-		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
+		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
+			propagate_yield_cpu(node, val, &set_yield_cpu, paravirt);
 			yield_head_to_locked_owner(lock, val, paravirt, false);
+		}
 
 		/* If we're the last queued, must clean up the tail. */
 		if ((val & _Q_TAIL_CPU_MASK) == tail) {
@@ -376,12 +440,14 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
 		/* We must be the owner, just set the lock bit and acquire */
 		lock_set_locked(lock);
 	} else {
+		int set_yield_cpu = -1;
 		int iters = 0;
 		bool set_mustq = false;
 
 again:
 		/* We're at the head of the waitqueue, wait for the lock. */
 		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
+			propagate_yield_cpu(node, val, &set_yield_cpu, paravirt);
 			yield_head_to_locked_owner(lock, val, paravirt,
 					pv_yield_allow_steal && set_mustq);
 
@@ -540,6 +606,22 @@ static int pv_yield_prev_get(void *data, u64 *val)
 
 DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_prev, pv_yield_prev_get, pv_yield_prev_set, "%llu\n");
 
+static int pv_yield_propagate_owner_set(void *data, u64 val)
+{
+	pv_yield_propagate_owner = !!val;
+
+	return 0;
+}
+
+static int pv_yield_propagate_owner_get(void *data, u64 *val)
+{
+	*val = pv_yield_propagate_owner;
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_propagate_owner, pv_yield_propagate_owner_get, pv_yield_propagate_owner_set, "%llu\n");
+
 static __init int spinlock_debugfs_init(void)
 {
 	debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_steal_spins);
@@ -548,6 +630,7 @@ static __init int spinlock_debugfs_init(void)
 		debugfs_create_file("qspl_pv_yield_owner", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_owner);
 		debugfs_create_file("qspl_pv_yield_allow_steal", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_allow_steal);
 		debugfs_create_file("qspl_pv_yield_prev", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_prev);
+		debugfs_create_file("qspl_pv_yield_propagate_owner", 0600, arch_debugfs_dir, NULL, &fops_pv_yield_propagate_owner);
 	}
 
 	return 0;
-- 
2.35.1