Subject: [PATCH 06/17] powerpc/qspinlock: theft prevention to control latency

From: Nicholas Piggin <[email-masked]>
Date: Thu, 28 Jul 2022 16:31:09 +1000

Give the queue head the ability to stop stealers. After a number of
spins without sucessfully acquiring the lock, the queue head employs
this, which will assure it is the next owner.
---
 arch/powerpc/include/asm/qspinlock_types.h | 10 +++-
 arch/powerpc/lib/qspinlock.c               | 56 +++++++++++++++++++++-
 2 files changed, 63 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/qspinlock_types.h b/arch/powerpc/include/asm/qspinlock_types.h
index 210adf05b235..8b20f5e22bba 100644
--- a/arch/powerpc/include/asm/qspinlock_types.h
+++ b/arch/powerpc/include/asm/qspinlock_types.h
@@ -29,7 +29,8 @@ typedef struct qspinlock {
  * Bitfields in the lock word:
  *
  *     0: locked bit
- * 16-31: tail cpu (+1)
+ *    16: must queue bit
+ * 17-31: tail cpu (+1)
  */
 #define	_Q_SET_MASK(type)	(((1U << _Q_ ## type ## _BITS) - 1)\
 				      << _Q_ ## type ## _OFFSET)
@@ -38,7 +39,12 @@ typedef struct qspinlock {
 #define _Q_LOCKED_MASK		_Q_SET_MASK(LOCKED)
 #define _Q_LOCKED_VAL		(1U << _Q_LOCKED_OFFSET)
 
-#define _Q_TAIL_CPU_OFFSET	16
+#define _Q_MUST_Q_OFFSET	16
+#define _Q_MUST_Q_BITS		1
+#define _Q_MUST_Q_MASK		_Q_SET_MASK(MUST_Q)
+#define _Q_MUST_Q_VAL		(1U << _Q_MUST_Q_OFFSET)
+
+#define _Q_TAIL_CPU_OFFSET	17
 #define _Q_TAIL_CPU_BITS	(32 - _Q_TAIL_CPU_OFFSET)
 #define _Q_TAIL_CPU_MASK	_Q_SET_MASK(TAIL_CPU)
 
diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 1625cce714b2..a906cc8f15fa 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -22,6 +22,7 @@ struct qnodes {
 /* Tuning parameters */
 static int STEAL_SPINS __read_mostly = (1<<5);
 static bool MAYBE_STEALERS __read_mostly = true;
+static int HEAD_SPINS __read_mostly = (1<<8);
 
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 
@@ -30,6 +31,11 @@ static __always_inline int get_steal_spins(void)
 	return STEAL_SPINS;
 }
 
+static __always_inline int get_head_spins(void)
+{
+	return HEAD_SPINS;
+}
+
 static inline u32 encode_tail_cpu(void)
 {
 	return (smp_processor_id() + 1) << _Q_TAIL_CPU_OFFSET;
@@ -142,6 +148,23 @@ static __always_inline u32 publish_tail_cpu(struct qspinlock *lock, u32 tail)
 	return prev;
 }
 
+static __always_inline u32 lock_set_mustq(struct qspinlock *lock)
+{
+	u32 new = _Q_MUST_Q_VAL;
+	u32 prev;
+
+	asm volatile(
+"1:	lwarx	%0,0,%1		# lock_set_mustq			\n"
+"	or	%0,%0,%2						\n"
+"	stwcx.	%0,0,%1							\n"
+"	bne-	1b							\n"
+	: "=&r" (prev)
+	: "r" (&lock->val), "r" (new)
+	: "cr0", "memory");
+
+	return prev;
+}
+
 static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
 {
 	int cpu = get_tail_cpu(val);
@@ -165,6 +188,9 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
 	for (;;) {
 		u32 val = READ_ONCE(lock->val);
 
+		if (val & _Q_MUST_Q_VAL)
+			break;
+
 		if (unlikely(!(val & _Q_LOCKED_VAL))) {
 			if (trylock_with_tail_cpu(lock, val))
 				return true;
@@ -246,11 +272,22 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
 		/* We must be the owner, just set the lock bit and acquire */
 		lock_set_locked(lock);
 	} else {
+		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)
+		while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
 			cpu_relax();
 
+			iters++;
+			if (!set_mustq && iters >= get_head_spins()) {
+				set_mustq = true;
+				lock_set_mustq(lock);
+				val |= _Q_MUST_Q_VAL;
+			}
+		}
+
 		/* If we're the last queued, must clean up the tail. */
 		if ((val & _Q_TAIL_CPU_MASK) == tail) {
 			if (trylock_clear_tail_cpu(lock, val))
@@ -329,9 +366,26 @@ 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 head_spins_set(void *data, u64 val)
+{
+	HEAD_SPINS = val;
+
+	return 0;
+}
+
+static int head_spins_get(void *data, u64 *val)
+{
+	*val = HEAD_SPINS;
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_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);
 
 	return 0;
 }
-- 
2.35.1