On Fri, Oct 25, 2019 at 4:53 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote: > > On 25-10-19, 02:41, Rafael J. Wysocki wrote: > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > > > 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.
