Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Boqun Feng" <boqun.feng@gmail.com>,
	"Paul McKenney" <paulmck@linux.vnet.ibm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Maciej W. Rozycki" <macro@imgtec.com>,
	"David Daney" <ddaney@caviumnetworks.com>,
	"Måns Rullgård" <mans@mansr.com>,
	"Ralf Baechle" <ralf@linux-mips.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()
Date: Tue, 2 Feb 2016 19:30:38 +0000	[thread overview]
Message-ID: <20160202193037.GQ10166@arm.com> (raw)
In-Reply-To: <CA+55aFzNmkzKs7PS6O1MB3vWp0-NDdgxVFUQ-kGmkh5L1gk+4w@mail.gmail.com>

On Tue, Feb 02, 2016 at 10:06:36AM -0800, Linus Torvalds wrote:
> On Tue, Feb 2, 2016 at 9:51 AM, Will Deacon <will.deacon@arm.com> wrote:
> >
> > Given that the vast majority of weakly ordered architectures respect
> > address dependencies, I would expect all of them to be hurt if they
> > were forced to use barrier instructions instead, even those where the
> > microarchitecture is fairly strongly ordered in practice.
> 
> I do wonder if it would be all that noticeable, though. I don't think
> we've really had benchmarks.
> 
> For example, most of the RCU list traversal shows up on x86 - where
> loads are already acquires. But they show up not because of that, but
> because a RCU list traversal is pretty much always going to take the
> cache miss.
> 
> So it would actually be interesting to just try it - what happens to
> kernel-centric benchmarks (which are already fairly rare) on arm if we
> change the rcu_dereference() to be a smp_load_acquire()?
> 
> Because maybe nothing happens at all. I don't think we've ever tried it.

FWIW, and this is by no means conclusive, I hacked that up quickly and
ran hackbench a few times on the nearest idle arm64 system. The results
were consistently ~4% slower using acquire for rcu_dereference.

Will