Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Andrea Parri]

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