linux-kernel - Re: [PATCH V7 5/7] cpufreq: Register notifiers with the PM QoS framework

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Date:   Wed, 16 Oct 2019 10:43:33 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Pavel Machek <pavel@....cz>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH V7 5/7] cpufreq: Register notifiers with the PM QoS framework

On Wed, Oct 16, 2019 at 10:27 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 15-10-19, 23:50, Rafael J. Wysocki wrote:
> > On Tue, Oct 15, 2019 at 5:53 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> > > > - Update QoS framework with the knowledge of related CPUs, this has been pending
> > > >   until now from my side. And this is the thing we really need to do. Eventually
> > > >   we shall have only a single notifier list for all CPUs of a policy, at least
> > > >   for MIN/MAX frequencies.
> > >
> > > - Move the PM QoS requests and notifiers to the new policy CPU on all
> > > changes of that.  That is, when cpufreq_offline() nominates the new
> > > "leader", all of the QoS stuff for the policy needs to go to this one.
> >
> > Alas, that still will not work, because things like
> > acpi_processor_ppc_init() only work accidentally for one-CPU policies.
>
> I am not sure what problem you see here ? Can you please explain a bit more.

Never mind, sorry.  This is called for policy->cpu too.

> > Generally, adding such a PM QoS request to a non-policy CPU simply has
> > no effect until it becomes a policy CPU which may be never.
>
> I was thinking maybe we can read the constraints for all CPUs in the
> policy->cpus mask in cpufreq_set_policy() and so this part of the problem will
> just go away. The only part that would be left is to remove the QoS constraints
> properly.

That would be on the complicated side IMO.

> > It looks like using device PM QoS for cpufreq is a mistake in general
> > and what is needed is a struct pm_qos_constraints member in struct
> > cpufreq_policy and something like
> >
> > struct freq_pm_qos_request {
> >         enum freq_pm_qos_req_type type; /* min or max */
> >         struct plist_node pnode;
> >         struct cpufreq_policy *policy;
> > };
> >
> > Then, pm_qos_update_target() can be used for adding, updating and
> > removing requests.

I have patches implementing this idea, more or less, almost ready,
stay tuned. :-)