powerpc/qspinlock: paravirt yield to lock owner

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.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221126095932.1234527-8-npiggin@gmail.com

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;
 }
@@ -46,6 +49,11 @@ static inline int decode_tail_cpu(u32 val)
 	return (val >> _Q_TAIL_CPU_OFFSET) - 1;
 }
 
+static inline int get_owner_cpu(u32 val)
+{
+	return (val & _Q_OWNER_CPU_MASK) >> _Q_OWNER_CPU_OFFSET;
+}
+
 /*
  * Try to acquire the lock if it was not already locked. If the tail matches
  * mytail then clear it, otherwise leave it unchnaged. Return previous value.
@@ -150,7 +158,45 @@ 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 = 0;
 
@@ -168,16 +214,16 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
 			if (__queued_spin_trylock_steal(lock))
 				return true;
 		} else {
-			cpu_relax();
+			yield_to_locked_owner(lock, val, paravirt);
 		}
 
 		iters++;
-	} while (iters < get_steal_spins());
+	} while (iters < get_steal_spins(paravirt));
 
 	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;
@@ -235,12 +281,12 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
 		if (!(val & _Q_LOCKED_VAL))
 			break;
 
-		cpu_relax();
+		yield_to_locked_owner(lock, val, paravirt);
 		if (!maybe_stealers)
 			continue;
 		iters++;
 
-		if (!mustq && iters >= get_head_spins()) {
+		if (!mustq && iters >= get_head_spins(paravirt)) {
 			mustq = true;
 			set_mustq(lock);
 			val |= _Q_MUST_Q_VAL;
@@ -276,10 +322,20 @@ 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);
+	/*
+	 * This looks funny, but it induces the compiler to inline both
+	 * sides of the branch rather than share code as when the condition
+	 * is passed as the paravirt argument to the functions.
+	 */
+	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);
 
@@ -345,10 +401,29 @@ 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;
 }