On Wed, 2015-07-15 at 11:44 +0100, Will Deacon wrote: > 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... Sorry no. Currently we use lwsync in lock. You'll see isync in the code (PPC_ACQUIRE_BARRIER), but on most modern CPUs that is patched at runtime to lwsync. I benchmarked lwsync (current), isync (proposed at the time) and sync (just for comparison). The numbers above are going from lwsync -> sync, as I thought that was what Ben was talking about. Going from lwsync to isync was actually a small speedup on power8, which was surprising. > > 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. No I mean that sync is clearly a memory barrier. The issue with switching to isync in lock was that it's not a memory barrier per se, so we were not 100% confident in it. > If you went the way of sync in unlock, could you remove the conditional > SYNC_IO stuff? Yeah we could, it's just a conditional sync in unlock when mmio has been done. That would fix the problem with smp_mb__after_unlock_lock(), but not the original worry we had about loads happening before the SC in lock. cheers -- 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
