Messages in this thread | | | Date | Fri, 13 Nov 2020 12:28:46 +0000 | From | Ionela Voinescu <> | Subject | Re: [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs |
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Hi,
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 review, Ionela.
> -- > Catalin
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