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

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Message-Id: <20170706162412.GE2393@linux.vnet.ibm.com>
Date:   Thu, 6 Jul 2017 09:24:12 -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 06:10:47PM +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.

> The current spin_unlock_wait() is weaker. Most notably it will not (with
> exception of ARM64/PPC for other reasons) cause waits on other CPUs.

Agreed, weaker semantics allow more optimizations.  So use cases needing
only the weaker semantics should more readily show performance benefits.
But either way, we need compelling use cases, and I do not believe that
any of the existing spin_unlock_wait() calls are compelling.  Perhaps I
am confused, but I am not seeing it for any of them.

							Thanx, Paul