Hi Michael, On Wed, Jul 15, 2015 at 04:06:18AM +0100, Michael Ellerman wrote: > On Tue, 2015-07-14 at 08:31 +1000, Benjamin Herrenschmidt wrote: > > On Mon, 2015-07-13 at 13:15 +0100, Will Deacon wrote: > > > This didn't go anywhere last time I posted it, but here it is again. > > > I'd really appreciate some feedback from the PowerPC guys, especially as > > > to whether this change requires them to add an additional barrier in > > > arch_spin_unlock and what the cost of that would be. > > > > We'd have to turn the lwsync in unlock or the isync in lock into a full > > barrier. As it is, we *almost* have a full barrier semantic, but not > > quite, as in things can get mixed up inside spin_lock between the LL and > > the SC (things leaking in past LL and things leaking "out" up before SC > > and then getting mixed up in there). > > > > Michael, at some point you were experimenting a bit with that and tried > > to get some perf numbers of the impact that would have, did that > > solidify ? Otherwise, I'll have a look when I'm back next week. > > I was mainly experimenting with replacing the lwsync in lock with an isync. > > But I think you're talking about making it a full sync in lock. > > That was about +7% on p8, +25% on p7 and +88% on p6. Ok, so that's overhead incurred by moving from isync -> lwsync? The numbers look quite large... > We got stuck deciding whether isync was safe to use as a memory barrier, > because the wording in the arch is a bit vague. > > But if we're talking about a full sync then I think there is no question that's > OK and we should just do it. Is this because there's a small overhead from lwsync -> sync? Just want to make sure I follow your reasoning. If you went the way of sync in unlock, could you remove the conditional SYNC_IO stuff? Will -- 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
