From: Michael Ellerman <mpe@ellerman.id.au>
To: Will Deacon <will.deacon@arm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
Date: Thu, 16 Jul 2015 12:00:28 +1000 [thread overview]
Message-ID: <1437012028.28475.2.camel@ellerman.id.au> (raw)
In-Reply-To: <20150715104420.GD1005@arm.com>
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
next prev parent reply other threads:[~2015-07-16 2:00 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-13 12:15 [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Will Deacon
2015-07-13 13:09 ` Peter Hurley
2015-07-13 14:24 ` Will Deacon
2015-07-13 15:56 ` Peter Zijlstra
2015-07-13 13:11 ` Peter Zijlstra
2015-07-13 14:09 ` Will Deacon
2015-07-13 14:21 ` Will Deacon
2015-07-13 15:54 ` Peter Zijlstra
2015-07-13 17:50 ` Will Deacon
2015-07-13 20:20 ` Paul E. McKenney
2015-07-13 22:23 ` Peter Zijlstra
2015-07-13 23:04 ` Paul E. McKenney
2015-07-14 10:04 ` Will Deacon
2015-07-14 12:45 ` Paul E. McKenney
2015-07-14 12:51 ` Will Deacon
2015-07-14 14:00 ` Paul E. McKenney
2015-07-14 14:12 ` Will Deacon
2015-07-14 19:31 ` Paul E. McKenney
2015-07-15 1:38 ` Paul E. McKenney
2015-07-15 10:51 ` Will Deacon
2015-07-15 13:12 ` Paul E. McKenney
2015-07-24 11:31 ` Will Deacon
2015-07-24 15:30 ` Paul E. McKenney
2015-08-12 13:44 ` Will Deacon
2015-08-12 15:43 ` Paul E. McKenney
2015-08-12 17:59 ` Paul E. McKenney
2015-08-13 10:49 ` Will Deacon
2015-08-13 13:10 ` Paul E. McKenney
2015-08-17 4:06 ` Michael Ellerman
2015-08-17 6:15 ` Paul E. McKenney
2015-08-17 8:57 ` Will Deacon
2015-08-18 1:50 ` Michael Ellerman
2015-08-18 8:37 ` Will Deacon
2015-08-20 9:45 ` Michael Ellerman
2015-08-20 15:56 ` Will Deacon
2015-08-26 0:27 ` Paul E. McKenney
2015-08-26 4:06 ` Michael Ellerman
2015-07-13 18:23 ` Paul E. McKenney
2015-07-13 19:41 ` Peter Hurley
2015-07-13 20:16 ` Paul E. McKenney
2015-07-13 22:15 ` Peter Zijlstra
2015-07-13 22:43 ` Benjamin Herrenschmidt
2015-07-14 8:34 ` Peter Zijlstra
2015-07-13 22:53 ` Paul E. McKenney
2015-07-13 22:37 ` Benjamin Herrenschmidt
2015-07-13 22:31 ` Benjamin Herrenschmidt
2015-07-14 10:16 ` Will Deacon
2015-07-15 3:06 ` Michael Ellerman
2015-07-15 10:44 ` Will Deacon
2015-07-16 2:00 ` Michael Ellerman [this message]
2015-07-16 5:03 ` Benjamin Herrenschmidt
2015-07-16 5:14 ` Benjamin Herrenschmidt
2015-07-16 15:11 ` Paul E. McKenney
2015-07-16 22:54 ` Benjamin Herrenschmidt
2015-07-17 9:32 ` Will Deacon
2015-07-17 10:15 ` Peter Zijlstra
2015-07-17 12:40 ` Paul E. McKenney
2015-07-17 22:14 ` Benjamin Herrenschmidt
2015-07-20 13:39 ` Will Deacon
2015-07-20 13:48 ` Paul E. McKenney
2015-07-20 13:56 ` Will Deacon
2015-07-20 21:18 ` Benjamin Herrenschmidt
2015-07-22 16:49 ` Will Deacon
2015-07-22 16:49 ` Will Deacon
2015-07-22 16:49 ` Will Deacon
2015-09-01 2:57 ` Paul Mackerras
2015-07-15 14:18 ` Paul E. McKenney
2015-07-16 1:34 ` Michael Ellerman
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