Messages in this thread | | | Subject | Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance | From | Giovanni Gherdovich <> | Date | Tue, 08 Oct 2019 09:48:39 +0200 |
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On Thu, 2019-10-03 at 19:53 +0200, Rafael J. Wysocki wrote: > On Thursday, October 3, 2019 2:15:37 PM CEST Peter Zijlstra wrote: > > On Thu, Oct 03, 2019 at 12:27:52PM +0200, Rafael J. Wysocki wrote: > > > On Wednesday, October 2, 2019 2:29:25 PM CEST Giovanni Gherdovich wrote: > > > > +static bool turbo_disabled(void) > > > > +{ > > > > + u64 misc_en; > > > > + int err; > > > > + > > > > + err = rdmsrl_safe(MSR_IA32_MISC_ENABLE, &misc_en); > > > > + if (err) > > > > + return false; > > > > + > > > > + return (misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE); > > > > +} > > > > > > This setting may be updated by the platform firmware (BIOS) in some cases > > > (see kernel.org BZ 200759, for example), so in general checking it once > > > at the init time is not enough. > > > > Is there anything sane we can do if the BIOS frobs stuff like that under > > our feet? Other than yell bloody murder, that is? > > Sane? No, I don't think so. > > Now, in principle *something* could be done to fix things up in the _PPC > notify handler, but I guess we would just end up disabling the scale > invariance code altogether in those cases.
I'm looking at how to react to turbo being disabled at run time, assuming a _PPC notification is triggered in that case.
I don't think the correct action would be to disable scale invariance: if the turbo range is not available, then max frequency is max_P, and scale invariance can go on using that. The case max_freq=max_P is represented by arch_max_freq=1024 in this patch (because arch_max_freq=max_freq*1024/max_P).
Since the variable arch_max_freq is global to all CPUs, the fact that the _PPC notification is sent to just one CPU is not a concern: the CPU receiving the notif will set arch_max_freq=1024 (Srinivas was worried about this in another message).
This looks like a job for the ->update_limits callback you added to "struct cpufreq_driver" in response to the mentioned kernel.org BZ 200759. I see that only intel_pstate implements it, it's not clear to me yet if I'll have to give an ->update_limits to acpi_cpufreq as well to treat this case.
Giovanni
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