Re: [PATCH v2 0/9] Remove spin_unlock_wait()

[Alan Stern <stern@rowland.harvard.edu>]

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