On Thu, 6 Jul 2017, Peter Zijlstra wrote: > On Thu, Jul 06, 2017 at 12:49:12PM -0400, Alan Stern wrote: > > 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. > > So please don't do that. That'll create massive pain for RT. Also I > don't think it works. The whole point was that spin_unlock_wait() is > _cheaper_ than lock()+unlock(). If it gets to be more expensive there is > absolutely no point in using it. Of course; that is obvious. I was making a rhetorical point: You should not try to justify spin_unlock_wait() on the basis that it doesn't cause waits on other CPUs. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
