Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load_relaxed

On Tue, Mar 09, 2021 at 07:46:11AM -0800, Davidlohr Bueso wrote:

On Tue, 09 Mar 2021, Michal Such�nek wrote:

quoted

On Mon, Mar 08, 2021 at 05:59:50PM -0800, Davidlohr Bueso wrote:

quoted

49a7d46a06c3 (powerpc: Implement smp_cond_load_relaxed()) added
busy-waiting pausing with a preferred SMT priority pattern, lowering
the priority (reducing decode cycles) during the whole loop slowpath.

However, data shows that while this pattern works well with simple

                                             ^^^^^^^^^^^^^^^^^^^^^^

quoted

spinlocks, queued spinlocks benefit more being kept in medium priority,
with a cpu_relax() instead, being a low+medium combo on powerpc.

...

quoted

diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index aecfde829d5d..7ae29cfb06c0 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h

@@ -80,22 +80,6 @@ do {									\

	___p1;								\
 })

-#ifdef CONFIG_PPC64

Maybe it should be kept for the simple spinlock case then?

It is kept, note that simple spinlocks don't use smp_cond_load_relaxed,
but instead deal with the priorities in arch_spin_lock(), so it will
spin in low priority until it sees a chance to take the lock, where
it switches back to medium.

Indeed, thanks for the clarification.

Michal