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Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()

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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
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[RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-13 14:20 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Peter Zijlstra <peterz@infradead.org> - 2015-07-14 10:40 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-14 12:10 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-14 21:40 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 03:40 +0200
        Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-15 13:00 +0200
          Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 15:20 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-15 05:10 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-15 12:50 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-16 04:10 +0200
        Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-16 17:20 +0200
          Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-07-17 01:00 +0200
            Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-17 11:40 +0200
              Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Peter Zijlstra <peterz@infradead.org> - 2015-07-17 12:20 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-17 14:50 +0200
              Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-07-18 00:20 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-20 15:40 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-20 15:50 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-20 16:00 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 16:20 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-16 03:40 +0200

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