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

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Date:   Tue, 11 Sep 2018 11:34:56 +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

Hi Quentin,

> > 1. use of a single memory barrier
> > 
> >    Since we are already em_pd_mutex protected, i.e. there cannot be a
> >    concurrent writers, we can use one single memory barrier after the
> >    loop, i.e.
> > 
> >         for_each_cpu(cpu, span)
> >                 WRITE_ONCE()
> >         smp_wmb()
> > 
> >    which should be just enough to ensure that all other CPUs will see
> >    the pointer set once we release the mutex
> 
> Right, I'm actually wondering if the memory barrier is needed at all ...
> The mutex lock()/unlock() should already ensure the ordering I want no ?
> 
> WRITE_ONCE() should prevent load/store tearing with concurrent em_cpu_get(),
> and the release/acquire semantics of mutex lock/unlock should be enough to
> serialize the memory accesses of concurrent em_register_perf_domain() calls
> properly ...
> 
> Hmm, let me read memory-barriers.txt again.

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... ;-)  Please don't hesitate
to contact me/the LKMM maintainers if you need help with this.

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...).

  Andrea