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

[Davidlohr Bueso]

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

> On Mon, Mar 08, 2021 at 05:59:50PM -0800, Davidlohr Bueso wrote:
> > 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
>                                              ^^^^^^^^^^^^^^^^^^^^^^
> > spinlocks, queued spinlocks benefit more being kept in medium priority,
> > with a cpu_relax() instead, being a low+medium combo on powerpc.
> ...
> > 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.

Thanks,
Davidlohr