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Message-ID: <Pine.LNX.4.44L0.1707061247070.1581-100000@iolanthe.rowland.org>
Date: Thu, 6 Jul 2017 12:49:12 -0400 (EDT)
From: Alan Stern <stern@...land.harvard.edu>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc: Peter Zijlstra <peterz@...radead.org>,
David Laight <David.Laight@...LAB.COM>,
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Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()
On Thu, 6 Jul 2017, 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.
>
> > 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.
If somebody really wants the full spin_unlock_wait semantics and
doesn't want to interfere with other CPUs, wouldn't synchronize_sched()
or something similar do the job? It wouldn't be as efficient as
lock+unlock, but it also wouldn't affect other CPUs.
Alan Stern
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