linux-kernel - Re: [RFC PATCH v3 05/10] sched/topology: Reference the Energy Model of CPUs when available

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Message-ID: <20180619171316.GB29374@e108498-lin.cambridge.arm.com>
Date:   Tue, 19 Jun 2018 18:13:17 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joelaf@...gle.com,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v3 05/10] sched/topology: Reference the Energy Model
 of CPUs when available

On Tuesday 19 Jun 2018 at 18:20:42 (+0200), Peter Zijlstra wrote:
> Right, so I would not do that many things at once. Also be more explicit
> about what data structure, and why.

OK, I can probably split that patch in two smaller patches. One that
introduces and enables the static_key (or something else to replace it,
see my reply below); and another one to create the list of frequency
domains.

> That said, I think the whole for_each_freq_domain() thing as done is
> broken. You've completely ignored the arguments to
> partition_sched_domains(). What happens if you create partitions right
> along the frequency domains?
> 
> So you really want an argument to for_each_freq_domain() to indicate
> who's frequency domains you want to iterate. And then I think it's
> easiest if you hook into build_sched_domains() instead, because you
> really want a list per root_domain I suspect

Hmm right, the current implementation is broken with multiple root
domains... We rarely use that in mobile but the code should work
regardless I suppose.

I agree having one list per root_domain should be better. I'll change that
in v4. But I also think the idea of a global static_key is broken then.
We need a per root_domain thing as well. The SD_ASYM_CPUCAPACITY flag
might be set on one hierarchy and not the other for ex. Not sure if
attaching a static_key to each root_domain really makes sense though ...

Would replacing the static_key by a flag attached to the root_domain be
reasonable ? The CONFIG_ENERGY_MODEL config option could help us make
sure there is no performance impact when EAS can't be used with something
like this:

	static bool sched_energy_enabled(struct root_domain *rd)
	{
	#ifdef CONFIG_ENERGY_MODEL
		return rd->eas_enabled;
	#else
		return false;
	#endif
	}

Or maybe a sched_feat instead of that ifdefery ?

> (and an empty list if there is but one entry on).

Sorry but I didn't understand that ...

Thanks,
Quentin