On Thu, Aug 28, 2025, Zihuan Zhang wrote:
> > Hmm, this is technically buggy. __free() won't invoke put_cpufreq_policy()
> > until
> > policy goes out of scope, and so using __free() means the code is
> > effectively:
> >
> > if (IS_ENABLED(CONFIG_CPU_FREQ)) {
> > struct cpufreq_policy *policy;
> > int cpu;
> >
> > cpu = get_cpu();
> > policy = cpufreq_cpu_get(cpu);
> > if (policy && policy->cpuinfo.max_freq)
> > max_tsc_khz = policy->cpuinfo.max_freq;
> > put_cpu();
> >
> > if (policy)
> > cpufreq_cpu_put(policy);
> > }
...
> Yes, this will indeed change the execution order.
> Can you accept that?
No, because it's buggy.
> Personally, I don’t think it’s ideal either.
>
> if (IS_ENABLED(CONFIG_CPU_FREQ)) {
> int cpu;
> cpu = get_cpu();
> {
> struct cpufreq_policy *policy
> __free(put_cpufreq_policy) = cpufreq_cpu_get(cpu);
> if (policy && policy->cpuinfo.max_freq)
> max_tsc_khz = policy->cpuinfo.max_freq;
> }
> put_cpu();
>
> }
>
> Other places may also have the same issue,
>
> maybe we should consider introducing a macro to handle this properly,
> so that initialization and cleanup are well defined without changing
> the existing order unexpected.
>
> like this:
>
> #define WITH_CPUFREQ_POLICY(cpu) {\
>
> for(struct cpufreq_policy *policy __free(put_cpufreq_policy) = \
> cpufreq_cpu_get(cpu); \
> policy;)
>
> Then Use it:
>
> if (IS_ENABLED(CONFIG_CPU_FREQ)) {
> int cpu;
> cpu = get_cpu();
> WITH_CPUFREQ_POLICY(cpu){
> if (policy->cpuinfo.max_freq)
> max_tsc_khz = policy->cpuinfo.max_freq;
> }
> put_cpu();
This all feels very forced, in the sense that we have a shiny new tool and are
trying to use it everywhere without thinking critically about whether or not
doing so is actually an improvement.
At a glance, this is literally the only instance in the entire kernel where the
CPU to use is grabbed immediately before the policy.
$ git grep -B 20 cpufreq_cpu_get | grep -e get_cpu -e smp_processor_id
arch/x86/kvm/x86.c- cpu = get_cpu();
drivers/cpufreq/cppc_cpufreq.c-static int cppc_get_cpu_power(struct device
*cpu_dev,
drivers/cpufreq/cppc_cpufreq.c-static int cppc_get_cpu_cost(struct device
*cpu_dev, unsigned long KHz,
drivers/cpufreq/mediatek-cpufreq-hw.c-mtk_cpufreq_get_cpu_power(struct device
*cpu_dev, unsigned long *uW,
Probably because KVM's usage is rather bizarre and honestly kind of dumb. But
KVM has had this behavior for 15+ years, so as weird as it is, I'm not inclined
to change it without a really, really strong reason to do so, e.g. to iterate
over all CPUs or something.
So given that this is the only intance of the problem patter, I think it makes
sense to leave KVM as-is, and not spend a bunch of time trying to figure out how
to make KVM's usage play nice with __free().
