From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Alan Stern <stern@rowland.harvard.edu>,
Will Deacon <will.deacon@arm.com>,
Akira Yokosawa <akiyks@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Daniel Lustig <dlustig@nvidia.com>,
David Howells <dhowells@redhat.com>,
Jade Alglave <j.alglave@ucl.ac.uk>,
Luc Maranget <luc.maranget@inria.fr>,
Nick Piggin <npiggin@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Fri, 13 Jul 2018 21:56:50 +0200 [thread overview]
Message-ID: <20180713195650.GA2408@andrea> (raw)
In-Reply-To: <20180713164239.GZ2494@hirez.programming.kicks-ass.net>
On Fri, Jul 13, 2018 at 06:42:39PM +0200, Peter Zijlstra wrote:
>
> Hi Michael,
>
> On Fri, Jul 13, 2018 at 11:15:26PM +1000, Michael Ellerman wrote:
> > I reran some numbers today with some slightly updated tests.
> >
> > It varies quite a bit across machines and CPU revisions.
> >
> > On one I get:
> >
> > Lock/Unlock Time Time % Total Cycles Cycles Cycles Delta
> > lwsync/lwsync 79,290,859,955 100.0 % 290,160,065,087 145 -
> > lwsync/sync 104,903,703,237 132.3 % 383,966,199,430 192 47
> >
> > Another:
> >
> > Lock/Unlock Time Time % Total Cycles Cycles Cycles Delta
> > lwsync/lwsync 71,662,395,722 100.0 % 252,403,777,715 126 -
> > lwsync/sync 84,932,987,977 118.5 % 299,141,951,285 150 23
> >
> >
> > So 18-32% slower, or 23-47 cycles.
>
> Very good info. Note that another option is to put the SYNC in lock() it
> doesn't really matter which of the two primitives gets it. I don't
> suppose it really matters for timing either way around.
>
> > Next week I can do some macro benchmarks, to see if it's actually
> > detectable at all.
> >
> > The other question is how they behave on a heavily loaded system.
> >
> >
> > My personal preference would be to switch to sync, we don't want to be
> > the only arch finding (or not finding!) exotic ordering bugs.
> >
> > But we'd also rather not make our slow locks any slower than they have
> > to be.
>
> I completely understand, but I'll get you beer (lots) if you do manage
> to make SYNC happen :-) :-)
One trivia about seems due: it's of course very easy to stick a full
or a "tso" fence in one's spin_lock() implementation, or to tight the
semantics of such a primitive; removing this fence, or weakening the
semantics is another matter...
(/me reminding about that spin_is_locked() discussion...)
Andrea
next prev parent reply other threads:[~2018-07-13 19:57 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-09 20:01 [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Alan Stern
2018-07-09 21:45 ` Paul E. McKenney
2018-07-10 13:57 ` Alan Stern
2018-07-10 16:25 ` Paul E. McKenney
[not found] ` <Pine.LNX.4.44L0.1807101416390.1449-100000@iolanthe.rowland.org>
2018-07-10 19:58 ` [PATCH v3] " Paul E. McKenney
2018-07-10 20:24 ` Alan Stern
2018-07-10 20:31 ` Paul E. McKenney
2018-07-11 9:43 ` Will Deacon
2018-07-11 15:42 ` Paul E. McKenney
2018-07-11 16:17 ` Andrea Parri
2018-07-11 18:03 ` Paul E. McKenney
2018-07-11 16:34 ` Peter Zijlstra
2018-07-11 18:10 ` Paul E. McKenney
2018-07-10 9:38 ` [PATCH v2] " Andrea Parri
2018-07-10 14:48 ` Alan Stern
2018-07-10 15:24 ` Andrea Parri
2018-07-10 15:34 ` Alan Stern
2018-07-10 23:14 ` Andrea Parri
2018-07-11 9:43 ` Will Deacon
2018-07-11 12:34 ` Andrea Parri
2018-07-11 12:54 ` Andrea Parri
2018-07-11 15:57 ` Will Deacon
2018-07-11 16:28 ` Andrea Parri
2018-07-11 17:00 ` Peter Zijlstra
2018-07-11 17:50 ` Daniel Lustig
2018-07-12 8:34 ` Andrea Parri
2018-07-12 9:29 ` Peter Zijlstra
2018-07-12 7:40 ` Peter Zijlstra
2018-07-12 9:34 ` Peter Zijlstra
2018-07-12 9:45 ` Will Deacon
2018-07-13 2:17 ` Daniel Lustig
2018-07-12 11:52 ` Andrea Parri
2018-07-12 12:01 ` Andrea Parri
2018-07-12 12:11 ` Peter Zijlstra
2018-07-12 13:48 ` Peter Zijlstra
2018-07-12 16:19 ` Paul E. McKenney
2018-07-12 17:04 ` Alan Stern
2018-07-12 17:14 ` Will Deacon
2018-07-12 17:28 ` Paul E. McKenney
2018-07-12 18:05 ` Peter Zijlstra
2018-07-12 18:10 ` Linus Torvalds
2018-07-12 19:52 ` Andrea Parri
2018-07-12 20:24 ` Andrea Parri
2018-07-13 2:05 ` Daniel Lustig
2018-07-13 4:03 ` Paul E. McKenney
2018-07-13 9:07 ` Andrea Parri
2018-07-13 9:35 ` Will Deacon
2018-07-13 17:16 ` Linus Torvalds
2018-07-13 19:06 ` Andrea Parri
2018-07-14 1:51 ` Alan Stern
2018-07-14 2:58 ` Linus Torvalds
2018-07-16 2:31 ` Paul E. McKenney
2018-07-13 11:08 ` Peter Zijlstra
2018-07-13 13:15 ` Michael Ellerman
2018-07-13 16:42 ` Peter Zijlstra
2018-07-13 19:56 ` Andrea Parri [this message]
2018-07-16 14:40 ` Michael Ellerman
2018-07-16 19:01 ` Peter Zijlstra
2018-07-16 19:30 ` Linus Torvalds
2018-07-17 14:45 ` Michael Ellerman
2018-07-17 16:19 ` Linus Torvalds
2018-07-17 18:33 ` Paul E. McKenney
2018-07-17 18:42 ` Peter Zijlstra
2018-07-17 19:40 ` Paul E. McKenney
2018-07-17 19:47 ` Alan Stern
2018-07-17 18:44 ` Linus Torvalds
2018-07-17 18:49 ` Linus Torvalds
2018-07-17 19:42 ` Paul E. McKenney
2018-07-17 19:37 ` Alan Stern
2018-07-17 20:13 ` Linus Torvalds
2018-07-17 19:38 ` Paul E. McKenney
2018-07-17 19:40 ` Andrea Parri
2018-07-17 19:52 ` Paul E. McKenney
2018-07-18 12:31 ` Michael Ellerman
2018-07-18 13:16 ` Michael Ellerman
2018-07-12 17:52 ` Andrea Parri
2018-07-12 20:43 ` Alan Stern
2018-07-12 21:13 ` Andrea Parri
2018-07-12 21:23 ` Andrea Parri
2018-07-12 18:33 ` Peter Zijlstra
2018-07-12 17:45 ` Andrea Parri
2018-07-10 16:56 ` Daniel Lustig
[not found] ` <Pine.LNX.4.44L0.1807101315140.1449-100000@iolanthe.rowland.org>
2018-07-10 23:31 ` Andrea Parri
2018-07-11 14:19 ` Alan Stern
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