Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 9 Mar 2021 18:30:26 +0100
From:   Michal =?iso-8859-1?Q?Such=E1nek?= <msuchanek@suse.de>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     npiggin@gmail.com, Davidlohr Bueso <dbueso@suse.de>,
        peterz@infradead.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, mingo@redhat.com, paulus@samba.org,
        longman@redhat.com, will@kernel.org
Subject: Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load_relaxed
Message-ID: <20210309173026.GB6564@kitsune.suse.cz>
References: <20210309015950.27688-1-dave@stgolabs.net>
 <20210309015950.27688-4-dave@stgolabs.net>
 <20210309093912.GW6564@kitsune.suse.cz>
 <20210309154611.kbxzx65auzvmfqnt@offworld>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20210309154611.kbxzx65auzvmfqnt@offworld>
User-Agent: Mutt/1.11.3 (2019-02-01)
Precedence: bulk

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

Indeed, thanks for the clarification.

Michal