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

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Date:   Tue, 11 Sep 2018 13:32:50 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Andrea Parri <andrea.parri@...rulasolutions.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

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 ?

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 ... :-)

Thanks,
Quentin