On Thu Nov 10, 2022 at 10:42 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

Having all CPUs poll the lock word for the owner CPU that should be
yielded to defeats most of the purpose of using MCS queueing for
scalability. Yet it may be desirable for queued waiters to to yield
to a preempted owner.

s390 addreses this problem by having queued waiters sample the lock
word to find the owner much less frequently. In this approach, the
waiters never sample it directly, but the queue head propagates the
owner CPU back to the next waiter if it ever finds the owner has
been preempted. Queued waiters then subsequently propagate the owner
CPU back to the next waiter, and so on.

Disable this option by default for now, i.e., no logical change.
---
 arch/powerpc/lib/qspinlock.c | 85 +++++++++++++++++++++++++++++++++++-
 1 file changed, 84 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 94f007f66942..28c85a2d5635 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c

@@ -12,6 +12,7 @@
 struct qnode {
 	struct qnode	*next;
 	struct qspinlock *lock;
+	int		yield_cpu;
 	u8		locked; /* 1 if lock acquired */
 };
 

@@ -28,6 +29,7 @@ static int HEAD_SPINS __read_mostly = (1<<8);
 static bool pv_yield_owner __read_mostly = true;
 static bool pv_yield_allow_steal __read_mostly = false;
 static bool pv_yield_prev __read_mostly = true;
+static bool pv_yield_propagate_owner __read_mostly = true;

This also seems to be enabled by default.

quoted

 
 static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
 

@@ -257,13 +259,66 @@ static __always_inline void yield_head_to_locked_owner(struct qspinlock *lock, u
 	__yield_to_locked_owner(lock, val, paravirt, clear_mustq);
 }
 
+static __always_inline void propagate_yield_cpu(struct qnode *node, u32 val, int *set_yield_cpu, bool paravirt)
+{
+	struct qnode *next;
+	int owner;
+
+	if (!paravirt)
+		return;
+	if (!pv_yield_propagate_owner)
+		return;
+
+	owner = get_owner_cpu(val);
+	if (*set_yield_cpu == owner)
+		return;
+
+	next = READ_ONCE(node->next);
+	if (!next)
+		return;
+
+	if (vcpu_is_preempted(owner)) {

Is there a difference about using vcpu_is_preempted() here
vs checking bit 0 in other places?

Yeah we weren't yielding here so didn't have to load the yield count
explicitly.

quoted

+		next->yield_cpu = owner;
+		*set_yield_cpu = owner;
+	} else if (*set_yield_cpu != -1) {

It might be worth giving the -1 CPU a #define.

It's fairly standard to use -1 as a null value for cpu.

quoted

+		next->yield_cpu = owner;
+		*set_yield_cpu = owner;
+	}
+}

Does this need to pass set_yield_cpu by reference? Couldn't it's new value be
returned? To me it makes it more clear the function is used to change
set_yield_cpu. I think this would work:

int set_yield_cpu = -1;

static __always_inline int propagate_yield_cpu(struct qnode *node, u32 val, int set_yield_cpu, bool paravirt)
{
	struct qnode *next;
	int owner;

	if (!paravirt)
		goto out;
	if (!pv_yield_propagate_owner)
		goto out;

	owner = get_owner_cpu(val);
	if (set_yield_cpu == owner)
		goto out;

	next = READ_ONCE(node->next);
	if (!next)
		goto out;

	if (vcpu_is_preempted(owner)) {
		next->yield_cpu = owner;
		return owner;
	} else if (set_yield_cpu != -1) {
		next->yield_cpu = owner;
		return owner;
	}

out:
	return set_yield_cpu;
}

set_yield_cpu = propagate_yield_cpu(...  set_yield_cpu ...);

I think I prefer as is, because the caller doesn't use it anywhere. It
looks more like some temporary storage to be used by the function over
multiple calls this way.

Thanks,
Nick