[REPOST PATCH v6 3/3] arm64: topology: Handle AMU FIE setup on CPU hotplug

[Geert Uytterhoeven]

Hi Beata,

On Tue, 13 Jan 2026 at 16:58, Beata Michalska <beata.michalska at arm.com> wrote:
> On Tue, Jan 13, 2026 at 11:51:45AM +0100, Geert Uytterhoeven wrote:
> > On Tue, 30 Dec 2025 at 09:02, Lifeng Zheng <zhenglifeng1 at huawei.com> wrote:
> > > Currently, when a cpufreq policy is created, the AMU FIE setup process
> > > checks all CPUs in the policy -- including those that are offline. If any
> > > of these CPUs are offline at that time, their AMU capability flag hasn't
> > > been verified yet, leading the check fail. As a result, AMU FIE is not
> > > enabled, even if the CPUs that are online do support it.
> > >
> > > Later, when the previously offline CPUs come online and report AMU support,
> > > there's no mechanism in place to re-enable AMU FIE for the policy. This
> > > leaves the entire frequency domain without AMU FIE, despite being eligible.
> > >
> > > Restrict the initial AMU FIE check to only those CPUs that are online at
> > > the time the policy is created, and allow CPUs that come online later to
> > > join the policy with AMU FIE enabled.
> > >
> > > Signed-off-by: Lifeng Zheng <zhenglifeng1 at huawei.com>
> > > Acked-by: Beata Michalska <beata.michalska at arm.com>
> >
> > Thanks for your patch, which is now commit 6fd9be0b7b2e957d
> > ("arm64: topology: Handle AMU FIE setup on CPU hotplug") in
> > arm64/for-next/core (next-20260107 and later).
> >
> > > --- a/arch/arm64/kernel/topology.c
> > > +++ b/arch/arm64/kernel/topology.c
> > > @@ -284,7 +284,7 @@ static int init_amu_fie_callback(struct notifier_block *nb, unsigned long val,
> > >         struct cpufreq_policy *policy = data;
> > >
> > >         if (val == CPUFREQ_CREATE_POLICY)
> > > -               amu_fie_setup(policy->related_cpus);
> > > +               amu_fie_setup(policy->cpus);
> > >
> > >         /*
> > >          * We don't need to handle CPUFREQ_REMOVE_POLICY event as the AMU
> > > @@ -303,10 +303,71 @@ static struct notifier_block init_amu_fie_notifier = {
> > >         .notifier_call = init_amu_fie_callback,
> > >  };
> > >
> > > +static int cpuhp_topology_online(unsigned int cpu)
> > > +{
> > > +       struct cpufreq_policy *policy = cpufreq_cpu_policy(cpu);
> > > +
> > > +       /* Those are cheap checks */
> > > +
> > > +       /*
> > > +        * Skip this CPU if:
> > > +        *  - it has no cpufreq policy assigned yet,
> > > +        *  - no policy exists that spans CPUs with AMU counters, or
> > > +        *  - it was already handled.
> > > +        */
> > > +       if (unlikely(!policy) || !cpumask_available(amu_fie_cpus) ||
> > > +           cpumask_test_cpu(cpu, amu_fie_cpus))
> > > +               return 0;
> > > +
> > > +       /*
> > > +        * Only proceed if all already-online CPUs in this policy
> > > +        * support AMU counters.
> > > +        */
> > > +       if (unlikely(!cpumask_subset(policy->cpus, amu_fie_cpus)))
> > > +               return 0;
> > > +
> > > +       /*
> > > +        * If the new online CPU cannot pass this check, all the CPUs related to
> > > +        * the same policy should be clear from amu_fie_cpus mask, otherwise they
> > > +        * may use different source of the freq scale.
> > > +        */
> > > +       if (!freq_counters_valid(cpu)) {
> > > +               pr_warn("CPU[%u] doesn't support AMU counters\n", cpu);
> >
> > This is triggered during resume from s2ram on Renesas R-Car H3
> > (big.LITTLE 4x Cortex-A57 + 4x Cortex-A53), when enabling the first
> > little core:
> >
> >     AMU: CPU[4] doesn't support AMU counters
> >
> > Adding debug code:
> >
> >     pr_info("Calling
> > topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, %*pbl)\n",
> > cpumask_pr_args(policy->related_cpus));
> >     pr_info("Calling cpumask_andnot(..., %*pbl, %*pbl)\n",
> > cpumask_pr_args(amu_fie_cpus), cpumask_pr_args(policy->related_cpus));
> >
> > gives:
> >
> >     AMU: Calling topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, 4-7)
> >     AMU: Calling cpumask_andnot(..., , 4-7)
> >
> > so AMU is disabled for all little cores.
> >
> > Since this only happens during s2ram, and not during initial CPU
> > bring-up on boot, this looks wrong to me?
> This does look rather surprising. If that CPU was marked as supporting AMUs at
> the initial bring-up it should be part of amu_fie_cpus mask, so the hp callback
> should bail out straight away. Would you be able to add some logs to see what
> that mask actually contains ?
> Furthermore, freq_counters_valid is logging issues when validating the counters.
> Would you be able to re-run it with the debug level to see what might be
> happening under the hood, although I am still unsure why it is even reaching
> that point ...

Adding extra debugging info, and "#define DEBUG" at the top.

During boot:

    AMU: amu_fie_setup:260: cpus 0-3 amu_fie_cpus
    ^^^ empty amu_fie_cpus
    AMU: CPU0: counters are not supported.
    ^^^ pr_debug
    AMU: amu_fie_setup:260: cpus 4-7 amu_fie_cpus
    ^^^ empty amu_fie_cpus
    AMU: CPU4: counters are not supported.
    ^^^ pr_debug

During resume from s2ram:

    AMU: cpuhp_topology_online:314: cpu 1 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))
    AMU: cpuhp_topology_online:314: cpu 2 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))
    AMU: cpuhp_topology_online:314: cpu 3 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))
    AMU: cpuhp_topology_online:314: cpu 4 amu_fie_cpus
    AMU: CPU4: counters are not supported.
    ^^^ pr_debug
    AMU: CPU[4] doesn't support AMU counters
    ^^^ pr_warn
    AMU: Calling topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, 4-7)
    AMU: Calling cpumask_andnot(..., , 4-7)
    AMU: cpuhp_topology_online:314: cpu 5 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))
    AMU: cpuhp_topology_online:314: cpu 6 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))
    AMU: cpuhp_topology_online:314: cpu 7 amu_fie_cpus
    AMU: cpuhp_topology_online:343: skipped
(!cpumask_subset(policy->cpus, amu_fie_cpus))

Hence there is no issue, as AMU is not supported at all!

The confusing part is in the (absence of) logging.
If AMU is not supported, freq_counters_valid() uses:

     pr_debug("CPU%d: counters are not supported.\n", cpu);

which is typically not printed, unless DEBUG is enabled.

If freq_counters_valid() failed, the new cpuhp_topology_online() uses:

    pr_warn("CPU[%u] doesn't support AMU counters\n", cpu);

which is always printed.

Given freq_counters_valid() already prints a (debug) message, I think
the pr_warn() should just be removed.  Do you agree, or is there still
another incorrect check that should prevent getting this far?

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds