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Going back to the original patch (997c021abc6e) that >>> states: >>> >>> "However, this deferred update mechanism is unnecessary and introduces extra >>> overhead for non-PCC register spaces (e.g. System Memory or FFH), where >>> accessing the regs won't sleep and can be safely performed from the tick >>> context." >>> >>> Clearly, the AMU registers cannot be arbitrarily accessed without >>> blocking when accessed from one CPU to another, so either this >>> function is never called in a cross-cpu context (and the warning >>> should be removed), or the premises of this change are wrong. >>> >>> Which one is it? >> The function is also called in cross-CPU contexts, but only from >> process context (get_rate, sysfs show) with IRQs enabled and >> calling smp_call_function_single() is safe. In the tick path, >> cppc_scale_freq_tick() uses per_cpu(cppc_freq_inv, smp_processor_id()). >> So we always read the current CPU's counters and no cross-CPU access. >> >> The premise of 997c021abc6e applies to same-CPU accesses only: >> Reading the local CPU's AMU regs does not sleep and is safe from >> tick context. >> The irqs_disabled() WARN is still needed to guard against unsafe >> cross-CPU calls. This also returns '-EPERM' unlike the WARN inside >> smp_call_function_single(). So we fail instead of risking deadlock. > Hmm, so why isn't this structured such as: > > if (irqs_disabled()) { > /* XXX: Explain the tick case */ > if (WARN_ON_ONCE(cpu != smp_processor_id())) > return -EPERM; > func(val); > } else { > smp_call_function_single(cpu, func, val, 1); > } > > return 0; > > That way, the irqs-enabled case remains the same and doesn't need to > mess with preemption. > > Will I will do the change in v3 as suggested. Thankyou Sumit Gupta