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Date:   Tue, 19 Jun 2018 14:38:45 +0100
From:   Quentin Perret <quentin.perret@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     rjw@rjwysocki.net, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
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        edubezval@gmail.com, srinivas.pandruvada@linux.intel.com,
        currojerez@riseup.net, javi.merino@kernel.org
Subject: Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management
 framework
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On Tuesday 19 Jun 2018 at 15:23:38 (+0200), Peter Zijlstra wrote:
> On Tue, Jun 19, 2018 at 01:58:58PM +0100, Quentin Perret wrote:
> > On Tuesday 19 Jun 2018 at 13:34:08 (+0200), Peter Zijlstra wrote:
> > > On Mon, May 21, 2018 at 03:24:58PM +0100, Quentin Perret wrote:
> > > > +struct em_freq_domain *em_cpu_get(int cpu)
> > > > +{
> > > > +	struct em_freq_domain *fd;
> > > > +	unsigned long flags;
> > > > +
> > > > +	read_lock_irqsave(&em_data_lock, flags);
> > > > +	fd = per_cpu(em_data, cpu);
> > > > +	read_unlock_irqrestore(&em_data_lock, flags);
> > > 
> > > Why can't this use RCU? This is the exact thing read_locks are terrible
> > > at and RCU excells at.
> > 
> > So the idea was that clients (the scheduler for ex) can get a reference
> > to a frequency domain object once, and they're guaranteed it always
> > exists without asking for it again.
> > 
> > For example, my proposal was to have the scheduler (patch 05) build its
> > own private list of frequency domains on which it can iterate efficiently
> > in the wake-up path. If we protect this per_cpu variable with RCU, then
> > this isn't possible any-more. The scheduler will have to re-ask
> > em_cpu_get() at every wake-up, and that makes iterating over frequency
> > domains a whole lot more complex.
> > 
> > Does that make any sense ?
> 
> None what so ever... The lock doesn't guarantee stability any more than
> RCU does.
> 
> If you hand out the pointer and then drop the read-lock, the write-lock
> can proceed and change the pointer right after you.
> 
> The very easiest solution is to never change the data, as I think was
> suggested elsewhere in the thread. Construct the thing once and then
> never mutate.

This is what is done actually. We will never write twice in the per_cpu
array itself. One of the fields (the table) in the structure pointed from
the per_cpu array can change, but not the pointer on the structure
itself.

The only reason this lock is here is to ensure the atomicity of the
write happening in em_register_freq_domain. But that write can happen
only once, the first time the frequency domain is registered.

But maybe I could use something simpler than a lock in this case ?
Would WRITE_ONCE/READ_ONCE be enough to ensure that atomicity for
example ?

Thanks,
Quentin