linux-kernel - Re: [PATCH v6 03/14] PM: Introduce an Energy Model management framework

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Date:   Tue, 11 Sep 2018 15:31:51 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>, peterz@...radead.org,
        rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        valentin.schneider@....com, vincent.guittot@...aro.org,
        thara.gopinath@...aro.org, viresh.kumar@...aro.org,
        tkjos@...gle.com, joel@...lfernandes.org, smuckle@...gle.com,
        adharmap@...eaurora.org, skannan@...eaurora.org,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [PATCH v6 03/14] PM: Introduce an Energy Model management
 framework

On Tue, Sep 11, 2018 at 01:32:50PM +0100, Quentin Perret wrote:
> Hi Andrea,
> 
> On Tuesday 11 Sep 2018 at 11:34:56 (+0200), Andrea Parri wrote:
> > FYI, the directory "tools/memory-model/" provides an "automated
> > memory-barriers.txt": in short, you encode your "memory ordering
> > questions" into "litmus tests" to be passed to the tool/simulator;
> > the tool will then answer with "Yes/No" (plus other information).
> > 
> > Some preparation is required to set up and learn how to use the
> > LKMM tools, but once there, I expect them to be more "efficient"
> > than reading memory-barriers.txt... ;-)
> 
> Thanks for pointing this out, I'll have a look.
> 
> > Please don't hesitate
> > to contact me/the LKMM maintainers if you need help with this.
> 
> And thanks for that too.
> 
> > You'd need some info in order to write down a _well-formed litmus
> > test, e.g., matching barrier/synchronization and interested memory
> > accesses on the reader side (IAC, the replacement "store-release
> > -> store-once+smp_wmb" discussed above is suspicious...).
> 
> Regarding the disccusion above, I was actually planning on removing the
> smp_wmb entirely and rely on WRITE_ONCE + mutex_{un}lock here. Do you
> see something obviously wrong with that ?

As said in IRC: nothing I can currently see.


> 
> I guess the LKMM tools should give me the yes/no answer I want, but if
> that's a no, I'd also like to understand why ... :-)

That answer would be a little bit more involved ... ;-)  The file
Documentation/explanation.txt within the above mentioned directory
could be a good starting point; skimming through litmus-tests/ and
Documentation/recipes.txt could also provide some hints.

  Andrea


> 
> Thanks,
> Quentin