On Thu, Oct 08, 2015 at 01:16:38PM +0200, Peter Zijlstra wrote: > On Thu, Oct 08, 2015 at 02:50:36PM +1100, Michael Ellerman wrote: > > On Wed, 2015-10-07 at 08:25 -0700, Paul E. McKenney wrote: > > > > Currently, we do need smp_mb__after_unlock_lock() to be after the > > > acquisition on PPC -- putting it between the unlock and the lock > > > of course doesn't cut it for the cross-thread unlock/lock case. > > This ^, that makes me think I don't understand > smp_mb__after_unlock_lock. > > How is: > > UNLOCK x > smp_mb__after_unlock_lock() > LOCK y > > a problem? That's still a full barrier. The problem is that I need smp_mb__after_unlock_lock() to give me transitivity even if the UNLOCK happened on one CPU and the LOCK on another. For that to work, the smp_mb__after_unlock_lock() needs to be either immediately after the acquire (the current choice) or immediately before the release (which would also work from a purely technical viewpoint, but I much prefer the current choice). Or am I missing your point? > > > I am with Peter -- we do need the benchmark results for PPC. > > > > Urgh, sorry guys. I have been slowly doing some benchmarks, but time is not > > plentiful at the moment. > > > > If we do a straight lwsync -> sync conversion for unlock it looks like that > > will cost us ~4.2% on Anton's standard context switch benchmark. > > And that does not seem to agree with Paul's smp_mb__after_unlock_lock() > usage and would not be sufficient for the same (as of yet unexplained) > reason. > > Why does it matter which of the LOCK or UNLOCK gets promoted to full > barrier on PPC in order to become RCsc? You could do either. However, as I understand it, there is hardware for which bc;isync is faster than lwsync. For such hardware, it is cheaper to upgrade the unlock from lwsync to sync than to upgrade the lock from bc;isync to sync. If I recall correctly, the kernel rewrites itself at boot to select whichever of lwsync or bc;isync is better for the hardware at hand. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
