Subject: [PATCH 08/17] powerpc/qspinlock: paravirt yield to lock owner

From: Nicholas Piggin <[email-masked]>
Date: Thu, 28 Jul 2022 16:31:11 +1000

Waiters spinning on the lock word should yield to the lock owner if the
vCPU is preempted. This improves performance when the hypervisor has
oversubscribed physical CPUs.
---
 arch/powerpc/lib/qspinlock.c | 97 ++++++++++++++++++++++++++++++------
 1 file changed, 83 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index aa26cfe21f18..55286ac91da5 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -5,6 +5,7 @@
 #include <linux/percpu.h>
 #include <linux/smp.h>
 #include <asm/qspinlock.h>
+#include <asm/paravirt.h>
 
 #define MAX_NODES	4
 
@@ -24,14 +25,16 @@ static int STEAL_SPINS __read_mostly = (1<<5);
 static bool MAYBE_STEALERS __read_mostly = true;
 static int HEAD_SPINS __read_mostly = (1<<8);
 
+static bool pv_yield_owner __read_mostly = true;
+
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 
-static __always_inline int get_steal_spins(void)
+static __always_inline int get_steal_spins(bool paravirt)
 {
 	return STEAL_SPINS;
 }
 
-static __always_inline int get_head_spins(void)
+static __always_inline int get_head_spins(bool paravirt)
 {
 	return HEAD_SPINS;
 }
@@ -46,7 +49,11 @@ static inline int get_tail_cpu(u32 val)
 	return (val >> _Q_TAIL_CPU_OFFSET) - 1;
 }
 
-/* Take the lock by setting the bit, no other CPUs may concurrently lock it. */
+static inline int get_owner_cpu(u32 val)
+{
+	return (val & _Q_OWNER_CPU_MASK) >> _Q_OWNER_CPU_OFFSET;
+}
+
 /* Take the lock by setting the lock bit, no other CPUs will touch it. */
 static __always_inline void lock_set_locked(struct qspinlock *lock)
 {
@@ -180,7 +187,45 @@ static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
 	BUG();
 }
 
-static inline bool try_to_steal_lock(struct qspinlock *lock)
+static __always_inline void yield_to_locked_owner(struct qspinlock *lock, u32 val, bool paravirt)
+{
+	int owner;
+	u32 yield_count;
+
+	BUG_ON(!(val & _Q_LOCKED_VAL));
+
+	if (!paravirt)
+		goto relax;
+
+	if (!pv_yield_owner)
+		goto relax;
+
+	owner = get_owner_cpu(val);
+	yield_count = yield_count_of(owner);
+
+	if ((yield_count & 1) == 0)
+		goto relax; /* owner vcpu is running */
+
+	/*
+	 * Read the lock word after sampling the yield count. On the other side
+	 * there may a wmb because the yield count update is done by the
+	 * hypervisor preemption and the value update by the OS, however this
+	 * ordering might reduce the chance of out of order accesses and
+	 * improve the heuristic.
+	 */
+	smp_rmb();
+
+	if (READ_ONCE(lock->val) == val) {
+		yield_to_preempted(owner, yield_count);
+		/* Don't relax if we yielded. Maybe we should? */
+		return;
+	}
+relax:
+	cpu_relax();
+}
+
+
+static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool paravirt)
 {
 	int iters;
 
@@ -197,18 +242,18 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
 			continue;
 		}
 
-		cpu_relax();
+		yield_to_locked_owner(lock, val, paravirt);
 
 		iters++;
 
-		if (iters >= get_steal_spins())
+		if (iters >= get_steal_spins(paravirt))
 			break;
 	}
 
 	return false;
 }
 
-static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
+static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, bool paravirt)
 {
 	struct qnodes *qnodesp;
 	struct qnode *next, *node;
@@ -260,7 +305,7 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
 	if (!MAYBE_STEALERS) {
 		/* We're at the head of the waitqueue, wait for the lock. */
 		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
-			cpu_relax();
+			yield_to_locked_owner(lock, val, paravirt);
 
 		/* If we're the last queued, must clean up the tail. */
 		if ((val & _Q_TAIL_CPU_MASK) == tail) {
@@ -278,10 +323,10 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
 again:
 		/* We're at the head of the waitqueue, wait for the lock. */
 		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
-			cpu_relax();
+			yield_to_locked_owner(lock, val, paravirt);
 
 			iters++;
-			if (!set_mustq && iters >= get_head_spins()) {
+			if (!set_mustq && iters >= get_head_spins(paravirt)) {
 				set_mustq = true;
 				lock_set_mustq(lock);
 				val |= _Q_MUST_Q_VAL;
@@ -320,10 +365,15 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
 
 void queued_spin_lock_slowpath(struct qspinlock *lock)
 {
-	if (try_to_steal_lock(lock))
-		return;
-
-	queued_spin_lock_mcs_queue(lock);
+	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) && is_shared_processor()) {
+		if (try_to_steal_lock(lock, true))
+			return;
+		queued_spin_lock_mcs_queue(lock, true);
+	} else {
+		if (try_to_steal_lock(lock, false))
+			return;
+		queued_spin_lock_mcs_queue(lock, false);
+	}
 }
 EXPORT_SYMBOL(queued_spin_lock_slowpath);
 
@@ -382,10 +432,29 @@ static int head_spins_get(void *data, u64 *val)
 
 DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_set, "%llu\n");
 
+static int pv_yield_owner_set(void *data, u64 val)
+{
+	pv_yield_owner = !!val;
+
+	return 0;
+}
+
+static int pv_yield_owner_get(void *data, u64 *val)
+{
+	*val = pv_yield_owner;
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_owner, pv_yield_owner_get, pv_yield_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);
 	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);
+	}
 
 	return 0;
 }
-- 
2.35.1