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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH v6 13/14] sched/topology: Make Energy Aware Scheduling depend on schedutil
Date:   Fri, 07 Sep 2018 10:56:12 +0200
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On Friday, September 7, 2018 10:52:01 AM CEST Rafael J. Wysocki wrote:
> On Thursday, September 6, 2018 4:38:44 PM CEST Quentin Perret wrote:
> > Hi Rafael,
> > 
> > On Thursday 06 Sep 2018 at 11:18:55 (+0200), Rafael J. Wysocki wrote:
> > > I'm not a particular fan of notifiers to be honest and you don't need
> > > to add an extra chain just in order to be able to register a callback
> > > from a single user.
> > 
> > Right. I agree there are alternatives to using notifiers. I used them
> > because they're existing infrastructure, and because they let me do what
> > I want without too much troubles, which are two important points.
> > 
> > > That can be achieved with a single callback
> > > pointer too, but also you could just call a function exported by the
> > > scheduler directly from where in the cpufreq code it needs to be
> > > called.
> > 
> > Are you thinking about something comparable to what is done in
> > cpufreq_add_update_util_hook() (kernel/sched/cpufreq.c) for example ?
> > That would probably have the same drawback as my current implementation,
> > that is that the scheduler is notified of _all_ governor changes, not
> > only changes to/from sugov although this is the only thing we care about
> > for EAS.
> 
> Well, why don't you implement it as something like "if the governor changes
> from sugov to something else (or the other way around), call this function
> from the scheduler"?

That said, governors are stopped and started in a few cases other than just
changing the governor, so maybe you want the EAS side to be notified whenever
sugov is stopped and started after all?