Re: [External] Re: [PATCH v3] arm64: topology: add source check in arch_cpu_idle_enter()

[Beata Michalska <beata.michalska@arm.com>]

On Wed, Aug 12, 2026 at 01:02:37PM +0200, Beata Michalska wrote:
> On Wed, Aug 12, 2026 at 07:34:52AM +0000, Sean Wang1 wrote:
> > On Tus, Aug 11, 2026 at 06:03PM, Beata Michalska wrote:
> > 
> > > > --- a/arch/arm64/kernel/topology.c
> > > > +++ b/arch/arm64/kernel/topology.c
> > > > @@ -175,7 +175,8 @@ void arch_cpu_idle_enter(void)
> > > >
> > > >  	/* Kick in AMU update but only if one has not happened already */
> > > >  	if (housekeeping_cpu(cpu, HK_TYPE_TICK) &&
> > > > -
> > > time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu)))
> > > > +
> > > time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu))
> > > &&
> > > > +	    topology_is_scale_freq_source(SCALE_FREQ_SOURCE_ARCH, cpu))
> > > >  		amu_scale_freq_tick();
> > > I'm not entirely convinced you gained a lot by that.
> > > It's one additional check per each enter_idle for case where AMUs are the
> > > source vs 2 additional check when it is not.
> > > Will try to figure out smth less 'invasive'.
> > > 
> > 
> > First, I think that the rcu_read_lock_sched()/unlock() in
> > topology_is_scale_freq_source() is unnecessary. arch_cpu_idle_enter()
> > is called from do_idle() after local_irq_disable() at
> > kernel/sched/idle.c:340, which satisfies the rcu_sched grace period
> > requirement. This means we can call rcu_dereference_sched() directly
> > without explicit RCU lock.
> In this particular case RCU locking is not required, though you are exposing
> an API that might be used in other curcumstances, so the least we could do is
> document that.
> > 
> > I have two options to propose:
> > 
> > Option A: Keep the helper, but drop the explicit RCU lock
> > 
> >     bool topology_is_scale_freq_source(enum scale_freq_source source,
> >                                        unsigned int cpu)
> >     {
> >         struct scale_freq_data *sfd;
> >         sfd = rcu_dereference_sched(*per_cpu_ptr(&sft_data, cpu));
> >         return sfd && sfd->source == source;
> >     }
> > 
> > Option B: Drop the helper entirely, check directly in arch_cpu_idle_enter()
> > If a generic exported helper feels too invasive, we can do the
> > check locally within arch_cpu_idle_enter() without touching
> > drivers/base/arch_topology.c at all:
> > 
> >     if (housekeeping_cpu(cpu, HK_TYPE_TICK) &&
> >         time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu))) {
> >         struct scale_freq_data *sfd;
> >         sfd = rcu_dereference_sched(*this_cpu_ptr(&sft_data));
> >         if (sfd && sfd->source == SCALE_FREQ_SOURCE_ARCH)
> >             amu_scale_freq_tick();
> >     }
> > 
> > This keeps the change entirely in arm64 code and avoids adding
> > a new exported symbol. Which approach would you prefer?
> I do not mind this additional helper. Besides, not my place to either mind it
> or not.
> What I do mind is doing the check in the arch idle enter path. I would rather
> see some notification triggered when the source gets changed so that
> the previous sfd code can do some state transition that would avoid us having
> to run the check in the first place.
> Still pondering on that one.
> Preferably I would drop that 'tick' call from there completely, but apparently
> this was needed on some platforms to make AMU readings more reliable.

How about smth between the lines of:

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index b32f13358fbb1..aff33836488c8 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -250,6 +250,22 @@ int arch_freq_get_on_cpu(int cpu)
 	return freq;
 }
 
+static int amu_fie_source_notifier(struct notifier_block *nb,
+				   unsigned long event,
+				   void *data)
+{
+	const struct cpumask *cpus = data;
+
+	if (event == SCALE_FREQ_SOURCE_ARCH)
+		cpumask_andnot(amu_fie_cpus, amu_fie_cpus, cpus);
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block amu_fie_nb = {
+	.notifier_call = amu_fie_source_notifier,
+};
+
 static void amu_fie_setup(const struct cpumask *cpus)
 {
 	int cpu;
@@ -274,6 +290,8 @@ static void amu_fie_setup(const struct cpumask *cpus)
 
 	topology_set_scale_freq_source(&amu_sfd, cpus);
 
+	if (cpumask_weight(cpus) == cpumask_weight(amu_fie_cpus))
+		topology_register_scale_freq_source_notifier(&amu_fie_nb);
 	pr_debug("CPUs[%*pbl]: counters will be used for FIE.",
 		 cpumask_pr_args(cpus));
 }
@@ -339,9 +357,9 @@ static int cpuhp_topology_online(unsigned int cpu)
 	}
 
 	cpumask_set_cpu(cpu, amu_fie_cpus);
-
 	topology_set_scale_freq_source(&amu_sfd, cpumask_of(cpu));
-
+	if (cpumask_weight(amu_fie_cpus) == 1)
+		topology_register_scale_freq_source_notifier(&amu_fie_nb);
 	pr_debug("CPU[%u]: counter will be used for FIE.", cpu);
 
 	return 0;
diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 8c5e47c28d9a3..096430a99ee00 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -22,11 +22,14 @@
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/units.h>
+#include <linux/notifier.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/hw_pressure.h>
 
 static DEFINE_PER_CPU(struct scale_freq_data __rcu *, sft_data);
+static struct blocking_notifier_head scale_freq_source_change =
+	BLOCKING_NOTIFIER_INIT(scale_freq_source_change);
 static struct cpumask scale_freq_counters_mask;
 static bool scale_freq_invariant;
 DEFINE_PER_CPU(unsigned long, capacity_freq_ref) = 0;
@@ -67,6 +70,18 @@ static void update_scale_freq_invariant(bool status)
 	}
 }
 
+int topology_register_scale_freq_source_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&scale_freq_source_change, nb);
+}
+EXPORT_SYMBOL_GPL(topology_register_scale_freq_source_notifier);
+
+int topology_unregister_scale_freq_source_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&scale_freq_source_change, nb);
+}
+EXPORT_SYMBOL_GPL(topology_unregister_scale_freq_source_notifier);
+
 void topology_set_scale_freq_source(struct scale_freq_data *data,
 				    const struct cpumask *cpus)
 {
@@ -95,6 +110,7 @@ void topology_set_scale_freq_source(struct scale_freq_data *data,
 	rcu_read_unlock();
 
 	update_scale_freq_invariant(true);
+
 }
 EXPORT_SYMBOL_GPL(topology_set_scale_freq_source);
 
@@ -102,8 +118,11 @@ void topology_clear_scale_freq_source(enum scale_freq_source source,
 				      const struct cpumask *cpus)
 {
 	struct scale_freq_data *sfd;
+	cpumask_var_t cleared_mask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
 	int cpu;
 
+	zalloc_cpumask_var(&cleared_mask, GFP_KERNEL);
+
 	rcu_read_lock();
 
 	for_each_cpu(cpu, cpus) {
@@ -112,6 +131,8 @@ void topology_clear_scale_freq_source(enum scale_freq_source source,
 		if (sfd && sfd->source == source) {
 			rcu_assign_pointer(per_cpu(sft_data, cpu), NULL);
 			cpumask_clear_cpu(cpu, &scale_freq_counters_mask);
+			if (cpumask_available(cleared_mask))
+				cpumask_set_cpu(cpu, cleared_mask);
 		}
 	}
 
@@ -124,6 +145,10 @@ void topology_clear_scale_freq_source(enum scale_freq_source source,
 	synchronize_rcu();
 
 	update_scale_freq_invariant(false);
+
+	if (cpumask_available(cleared_mask))
+		blocking_notifier_call_chain(&scale_freq_source_change, source,
+					     cleared_mask);
 }
 EXPORT_SYMBOL_GPL(topology_clear_scale_freq_source);
 
diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
index ebd7f8935f969..4c31fd6dff0ef 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -48,6 +48,8 @@ struct scale_freq_data {
 void topology_scale_freq_tick(void);
 void topology_set_scale_freq_source(struct scale_freq_data *data, const struct cpumask *cpus);
 void topology_clear_scale_freq_source(enum scale_freq_source source, const struct cpumask *cpus);
+int topology_register_scale_freq_source_notifier(struct notifier_block *nb);
+int topology_unregister_scale_freq_source_notifier(struct notifier_block *nb);
 
 DECLARE_PER_CPU(unsigned long, hw_pressure);
 

---

This is just a rough idea, and needs ironing out the wrinkles, which are there,
but that allows leaving the idle enter as is. I also believe this is the right
approach for the interface itself, although I also see some drawbacks and
potential issues (in its current state), especially that the functionality is
being exposed to modules.
Nevertheless, those are my two cents, untested, just sketched.

---
BR
Beata
> 
> > 
> > > Aside: I should have probably asked that earlier, but I am not sure I do fully
> > > understand the case we are trying to fix here.
> > > The topology_set_scale_freq_source prefers arch source to others. So if the
> > > AMUs were chosen to server as the source for the freq scale - I do not see
> > > why the sfd would be changed. That would require calling sequence clear-set
> > > to get a different source in place. I do understand the issue itself, though how
> > > did we end up there in the first place ?
> > 
> > The issue arises when topology_clear_scale_freq_source() is called
> > with SCALE_FREQ_SOURCE_ARCH to explicitly disable AMU-based frequency
> > scaling. This API is exported (EXPORT_SYMBOL_GPL), so it is designed
> > to be used by modules or subsystems that need to replace the frequency
> > invariance mechanism at runtime.
> >  
> So this is the bit I was missing: external module that does the switch
> willingly giving up on arch provided freq scale source.
> The rest is clear. Thanks.
> 
> ---
> BR
> Beata
> 
> > After clearing, the tick path (topology_scale_freq_tick()) correctly
> > skips the AMU update because sft_data is set to NULL. However, the
> > idle path (arch_cpu_idle_enter()) bypasses this check by calling
> > amu_scale_freq_tick() directly, so arch_freq_scale still gets
> > modified by AMU counters.
> >  
> > This creates an inconsistency: the tick path respects
> > topology_clear_scale_freq_source() but the idle path does not.
> >  
> > The goal of this patch is to make the idle path consistent with
> > the tick path, ensuring that topology_clear_scale_freq_source()
> > fully disables AMU updates across all paths.