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

Allow new waiters a number of spins on the lock word before queueing,
which particularly helps paravirt performance when physical CPUs are
oversubscribed.
---
 arch/powerpc/lib/qspinlock.c | 152 ++++++++++++++++++++++++++++++++---
 1 file changed, 141 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 7c71e5e287df..1625cce714b2 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c

@@ -19,8 +19,17 @@ struct qnodes {
 	struct qnode nodes[MAX_NODES];
 };
 
+/* Tuning parameters */
+static int STEAL_SPINS __read_mostly = (1<<5);
+static bool MAYBE_STEALERS __read_mostly = true;

I can understand why, but macro case variables can be a bit confusing.

Yeah they started out as #defines. I'll change them.

quoted

+
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 
+static __always_inline int get_steal_spins(void)
+{
+	return STEAL_SPINS;
+}
+
 static inline u32 encode_tail_cpu(void)
 {
 	return (smp_processor_id() + 1) << _Q_TAIL_CPU_OFFSET;

@@ -76,6 +85,39 @@ static __always_inline int trylock_clear_tail_cpu(struct qspinlock *lock, u32 ol
 	return 0;
 }
 
+static __always_inline u32 __trylock_cmpxchg(struct qspinlock *lock, u32 old, u32 new)
+{
+	u32 prev;
+
+	BUG_ON(old & _Q_LOCKED_VAL);
+
+	asm volatile(
+"1:	lwarx	%0,0,%1,%4	# queued_spin_trylock_cmpxchg		\n"

s/queued_spin_trylock_cmpxchg/__trylock_cmpxchg/

Yes.

btw what is the format you using for the '\n's in the inline asm?

Ah, not really sure :P

quoted

+"	cmpw	0,%0,%2							\n"
+"	bne-	2f							\n"
+"	stwcx.	%3,0,%1							\n"
+"	bne-	1b							\n"
+"\t"	PPC_ACQUIRE_BARRIER "						\n"
+"2:									\n"
+	: "=&r" (prev)
+	: "r" (&lock->val), "r"(old), "r" (new),
+	  "i" (IS_ENABLED(CONFIG_PPC64) ? 1 : 0)
+	: "cr0", "memory");

This is very similar to trylock_clear_tail_cpu(). So maybe it is worth having
some form of "test and set" primitive helper.

Yes I was able to consolidate these two, good point.

quoted

+
+	return prev;
+}
+
+/* Take lock, preserving tail, cmpxchg with val (which must not be locked) */
+static __always_inline int trylock_with_tail_cpu(struct qspinlock *lock, u32 val)
+{
+	u32 newval = _Q_LOCKED_VAL | (val & _Q_TAIL_CPU_MASK);
+
+	if (__trylock_cmpxchg(lock, val, newval) == val)
+		return 1;
+	else
+		return 0;

same optional style nit: return __trylock_cmpxchg(lock, val, newval) == val

quoted

+}
+
 /*
  * Publish our tail, replacing previous tail. Return previous value.
  *

@@ -115,6 +157,31 @@ static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
 	BUG();
 }
 
+static inline bool try_to_steal_lock(struct qspinlock *lock)
+{
+	int iters;
+
+	/* Attempt to steal the lock */
+	for (;;) {
+		u32 val = READ_ONCE(lock->val);
+
+		if (unlikely(!(val & _Q_LOCKED_VAL))) {
+			if (trylock_with_tail_cpu(lock, val))
+				return true;
+			continue;
+		}

The continue would bypass iters++/cpu_relax but the next time around
  if (unlikely(!(val & _Q_LOCKED_VAL))) {
should fail so everything should be fine?

Yes it should. I suppose it could starve in theory though. Maybe
I'll change it to count as an iteration.

quoted

+#include <linux/debugfs.h>
+static int steal_spins_set(void *data, u64 val)
+{
+	static DEFINE_MUTEX(lock);

I just want to check if it would be possible to get rid of the MAYBE_STEALERS
variable completely and do something like:

  bool maybe_stealers() { return STEAL_SPINS > 0; }

I guess based on the below code it wouldn't work, but I'm still not quite sure
why that is.

Because the slowpath has a !maybe_stealers path which assumes the
lock won't be stolen so it doesn't need to cmpxchg the lock bit on,
among other things.

I'll add a bit more comment.

quoted

+
+	mutex_lock(&lock);
+	if (val && !STEAL_SPINS) {
+		MAYBE_STEALERS = true;
+		/* wait for waiter to go away */
+		synchronize_rcu();
+		STEAL_SPINS = val;
+	} else if (!val && STEAL_SPINS) {
+		STEAL_SPINS = val;
+		/* wait for all possible stealers to go away */
+		synchronize_rcu();
+		MAYBE_STEALERS = false;
+	} else {
+		STEAL_SPINS = val;
+	}
+	mutex_unlock(&lock);

STEAL_SPINS is an int not a u64.

Yeah but that's how the DEFINE_SIMPLE_ATTRIBUTE things seem to work,
unfortunately.

Thanks,
Nick