linux-kernel - Re: [PATCH] ACPI: processor: Add QoS requests for all CPUs

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Date:   Fri, 25 Oct 2019 10:17:37 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI: processor: Add QoS requests for all CPUs

On Fri, Oct 25, 2019 at 4:53 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 25-10-19, 02:41, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > The _PPC change notifications from the platform firmware are per-CPU,
> > so acpi_processor_ppc_init() needs to add a frequency QoS request
> > for each CPU covered by a cpufreq policy to take all of them into
> > account.
> >
> > Even though ACPI thermal control of CPUs sets frequency limits
> > per processor package, it also needs a frequency QoS request for each
> > CPU in a cpufreq policy in case some of them are taken offline and
> > the frequency limit needs to be set through the remaining online
> > ones (this is slightly excessive, because all CPUs covered by one
> > cpufreq policy will set the same frequency limit through their QoS
> > requests, but it is not incorrect).
> >
> > Modify the code in accordance with the above observations.
>
> I am not sure if I understood everything you just said, but I don't
> see how things can break with the current code we have.
>
> Both acpi_thermal_cpufreq_init() and acpi_processor_ppc_init() are
> called from acpi_processor_notifier() which is registered as a policy
> notifier and is called when a policy is created or removed. Even if
> some CPUs of a policy go offline, it won't matter as the request for
> the policy stays and it will be dropped only when all the CPUs of a
> policy go offline.
>
> What am I missing ?

The way the request is used.

Say there are two CPUs, A and B, in the same policy.  A is
policy->cpu, so acpi_processor_ppc_init() adds a QoS request for A
only (note that the B's QoS request, B->perflib_req, remains
inactive).

Now, some time later, the platform firmware notifies the OS of a _PPC
change for B.  That means acpi_processor_notify() is called and it
calls acpi_processor_ppc_has_changed(B) and that invokes
acpi_processor_get_platform_limit(B), which in turn looks at the B's
QoS request (B->perflib_req) and sees that it is inactive, so 0 is
returned without doing anything.  However, *some* QoS request should
be updated then.

Would it be correct to update the A's QoS request in that case?  No,
because the _PPC limit for A may be different that the _PPC limit for
B in principle.

The thermal case is not completely analogous, because
cpufreq_set_cur_state() finds online CPUs in the same package as the
target one and tries to update the QoS request for each of them, which
will include the original policy->cpu, whose QoS request has been
registered by acpi_thermal_cpufreq_init(), as long as it is online.
If it is offline, it will be skipped and there is no easy way to find
a "previous policy->cpu".  It is possible to do that, but IMO it is
more straightforward to have a request for each CPU added.