linux-kernel - Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load

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Message-ID: <20210309173026.GB6564@kitsune.suse.cz>
Date:   Tue, 9 Mar 2021 18:30:26 +0100
From:   Michal Suchánek <msuchanek@...e.de>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     npiggin@...il.com, Davidlohr Bueso <dbueso@...e.de>,
        peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, paulus@...ba.org,
        longman@...hat.com, will@...nel.org
Subject: 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 Suchï¿½nek 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.

Indeed, thanks for the clarification.

Michal