On Thu Nov 10, 2022 at 10:40 AM AEST, Jordan Niethe wrote:

On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
[resend as utf-8, not utf-7]

quoted

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)

Not a big deal but some of these values could be calculated like in the
generic version. e.g.

	#define _Q_PENDING_OFFSET	(_Q_LOCKED_OFFSET +_Q_LOCKED_BITS)

Yeah, we don't *really* have more than one locked bit though. Haven't
made up my mind about these defines yet.

quoted

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"

Is the EH bit not set because we don't hold the lock here?

Right, we're still waiting for it.

quoted

+"	or	%0,%0,%2						\n"
+"	stwcx.	%0,0,%1							\n"
+"	bne-	1b							\n"
+	: "=&r" (prev)
+	: "r" (&lock->val), "r" (new)
+	: "cr0", "memory");

This is another usage close to the DEFINE_TESTOP() pattern.

quoted

+
+	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++;

It seems instead of using set_mustq, (val & _Q_MUST_Q_VAL) could be checked?

I wanted to give the reader (and compiler for what that's worth) the
idea that it won't change concurrently after we set it.

Thanks,
Nick