linux-kernel - Re: [PATCH v7 08/14] sched/topology: Disable EAS on inappropriate platforms

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Date:   Thu, 4 Oct 2018 10:45:47 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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,
        patrick.bellasi@....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 v7 08/14] sched/topology: Disable EAS on inappropriate
 platforms

On Thursday 04 Oct 2018 at 11:38:48 (+0200), Peter Zijlstra wrote:
> On Thu, Oct 04, 2018 at 10:10:48AM +0100, Quentin Perret wrote:
> > On Wednesday 03 Oct 2018 at 18:27:19 (+0200), Peter Zijlstra wrote:
> > > On Wed, Sep 12, 2018 at 10:13:03AM +0100, Quentin Perret wrote:
> > > > @@ -288,6 +321,21 @@ static void build_perf_domains(const struct cpumask *cpu_map)
> > > >  			goto free;
> > > >  		tmp->next = pd;
> > > >  		pd = tmp;
> > > > +
> > > > +		/*
> > > > +		 * Count performance domains and capacity states for the
> > > > +		 * complexity check.
> > > > +		 */
> > > > +		nr_pd++;
> > > > +		nr_cs += em_pd_nr_cap_states(pd->obj);
> > > > +	}
> > > > +
> > > > +	/* Bail out if the Energy Model complexity is too high. */
> > > > +	if (nr_pd * (nr_cs + nr_cpus) > EM_MAX_COMPLEXITY) {
> > > > +		if (sched_debug())
> > > > +			pr_info("rd %*pbl: EM complexity is too high\n ",
> > > > +						cpumask_pr_args(cpu_map));
> > > > +		goto free;
> > > >  	}
> > > 
> > > I would make than an unconditional WARN, we do not really expect that to
> > > trigger, but then it does, we really don't want to hide it.
> > 
> > OTOH that also means that some people with big asymmetric machines can
> > get a WARN message every time they boot, and even if they don't want to
> > use EAS.
> > 
> > Now, that shouldn't happen any time soon, so it's maybe a good thing if
> > we get reports when/if people start to hit that one, so why not ...
> 
> Right, and if becomes a real problem we can think of a solution (like
> maybe a DT thingy that says to not use EAS, or a 'better' EAS
> algorithm).

That works for me. I'll switch to a plain WARN in v8.

Thanks,
Quentin