On Fri, Oct 25, 2019 at 10:17 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > 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. BTW, IMO processor_thremal can be changed to use one frequency QoS request per policy on top of this, but I'd rather take one step at a time. :-)
