Messages in this thread | | | Date | Fri, 13 Nov 2020 12:54:33 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs |
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On Fri, Nov 13, 2020 at 12:28:46PM +0000, Ionela Voinescu wrote: > On Thursday 12 Nov 2020 at 18:00:46 (+0000), Catalin Marinas wrote: > > On Fri, Nov 06, 2020 at 12:53:34PM +0000, Ionela Voinescu wrote: > > > +static inline > > > +int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val) > > > +{ > > > + if (!cpu_has_amu_feat(cpu)) > > > + return -EOPNOTSUPP; > > > + > > > + smp_call_function_single(cpu, func, val, 1); > > > + > > > + return 0; > > > +} > > > > I got lost in the cpufreq call chains. Can this function ever be called > > with interrupts disabled? > > The short answer is: not with the current implementation of its only > user, the cppc_cpufreq driver (given the current cpufreq implementation). > > The long answer is: there is a case where the cpufreq .get function is > called with local interrupts disabled - cpufreq_quick_get(), but there > are a few "if"s in between this becoming a problem: > > 1. If cppc_cpufreq ever implements .setpolicy or, > 1.1 Current implementation of cpufreq_quick_get() changes. > 2. If one of the few users of cpufreq_quick_get() is used: cppc_cpufreq > ends up being used as CPU cooling device(+IPA governor) or > devfreq/tegra30 is used. > > In this potential case, smp_call_function_single() will warn us of call > with irqs_disable() and if the stars align there could be a potential > deadlock if two CPUs try to IPI (get counter reads of) each other. > > So it could be called with irqs disabled, but a few code changes are > needed before that happens. > > I can bail out of counters_read_on_cpu() if irqs_disabled() to be on the > safe side.
Thanks for the explanation. In case we forget two years from now and the core code changes, I think it's safe to bail out early with an error. You can add a patch on top of this series, no need to resend the whole.
-- Catalin
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