linux-kernel - Re: [PATCH v2 0/9] Remove spin_unlock

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Message-Id: <20170706170302.GG2393@linux.vnet.ibm.com>
Date:   Thu, 6 Jul 2017 10:03:02 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "manfred@...orfullife.com" <manfred@...orfullife.com>,
        "tj@...nel.org" <tj@...nel.org>, "arnd@...db.de" <arnd@...db.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()

On Thu, Jul 06, 2017 at 06:41:34PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 06, 2017 at 09:24:12AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 06, 2017 at 06:10:47PM +0200, Peter Zijlstra wrote:
> > > On Thu, Jul 06, 2017 at 08:21:10AM -0700, Paul E. McKenney wrote:
> > > > And yes, there are architecture-specific optimizations for an
> > > > empty spin_lock()/spin_unlock() critical section, and the current
> > > > arch_spin_unlock_wait() implementations show some of these optimizations.
> > > > But I expect that performance benefits would need to be demonstrated at
> > > > the system level.
> > > 
> > > I do in fact contended there are any optimizations for the exact
> > > lock+unlock semantics.
> > 
> > You lost me on this one.
> 
> For the exact semantics you'd have to fully participate in the fairness
> protocol. You have to in fact acquire the lock in order to have the
> other contending CPUs wait (otherwise my earlier case 3 will fail).
> 
> At that point I'm not sure there is much actual code you can leave out.
> 
> What actual optimization is there left at that point?

Got it.  It was just that I was having a hard time parsing your sentence.
You were contending that there are no optimizations for all implementations
for the full semantics.

							Thanx, Paul