Date: Wed, 16 Mar 2016 08:47:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Michael Turquette <mturquette@baylibre.com>
Cc: rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Juri.Lelli@arm.com, steve.muckle@linaro.org, morten.rasmussen@arm.com,
        dietmar.eggemann@arm.com, vincent.guittot@linaro.org,
        Michael Turquette <mturquette+renesas@baylibre.com>
Subject: Re: [PATCH 8/8] sched: prefer cpufreq_scale_freq_capacity
Message-ID: <20160316074752.GQ6344@twins.programming.kicks-ass.net>
References: <1457932932-28444-1-git-send-email-mturquette+renesas@baylibre.com>
 <1457932932-28444-9-git-send-email-mturquette+renesas@baylibre.com>
 <20160315213744.GJ6344@twins.programming.kicks-ass.net>
 <20160315222721.30639.28332@quark.deferred.io>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160315222721.30639.28332@quark.deferred.io>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1369
Lines: 30

On Tue, Mar 15, 2016 at 03:27:21PM -0700, Michael Turquette wrote:

> That solution scales for the case where architectures have different
> methods. It doesn't scale for the case where cpufreq drivers or platform
> code within the same arch have competing implementations.

Sure it does; no matter what interface we use on x86 to set the DVFS
hints (ACPI, intel_p_state, whatever), using APERF/MPERF is the only
actual way of telling WTH the actual frequency was.

> I'm happy with it as a stop-gap, because it will initially work for
> arm{64} and x86, but we'll still need run-time selection of
> arch_scale_freq_capacity some day. Once we have that, I think that we
> should favor a run-time provided implementation over the arch-provided
> one.

Also, I'm thinking we don't need any of this. Your
cpufreq_scale_freq_capacity() is completely and utterly pointless. Since
its implementation simply provides whatever frequency we selected its
identical to not using frequency invariant load metrics and having
cpufreq use the !inv formula.

See:

  lkml.kernel.org/r/20160309163930.GP6356@twins.programming.kicks-ass.net

Now, something else (power aware scheduling etc..) might need the freq
invariant stuff, but cpufreq (which we're concerned with here) does not
unless arch_scale_freq_capacity() does something else than simply return
the value we've set earlier.