On 2026-03-20 09:21, Harry Yoo (Oracle) wrote:
On Fri, Mar 20, 2026 at 08:35:46AM -0400, Mathieu Desnoyers wrote:
On 2026-03-20 00:17, Harry Yoo wrote:
[...]
[1]:
https://lore.kernel.org/[email protected]/
@Mathieu: In patch 1/3 description,
Changes since v7:
- Explicitly initialize the subsystem from start_kernel() right
after mm_core_init() so it is up and running before the creation of
the first mm at boot.
But how does this work when someone calls mm_cpumask() on init_mm early?
Looks like it will behave incorrectly because get_rss_stat_items_size()
returns zero?
It doesn't work as expected at all. I missed that all users of mm_cpumask()
end up relying on get_rss_stat_items_size(), which now calls
percpu_counter_tree_items_size(), which depends on initialization from
percpu_counter_tree_subsystem_init().
If you add a call to percpu_counter_tree_subsystem_init in
arch/powerpc/kernel/setup_arch() just before:
VM_WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(&init_mm)));
cpumask_set_cpu(smp_processor_id(), mm_cpumask(&init_mm));
Does the warning go away ?
Hmm it goes away, but I'm not sure if it is it okay to use nr_cpu_ids
before setup_nr_cpu_ids() is called?
AFAIU on powerpc setup_nr_cpu_ids() is called near the end of
smp_setup_cpu_maps(), which is called early in setup_arch,
at least before the two lines which use mm_cpumask.
Alternatively, would could use a lazy initialization invoking
percpu_counter_tree_subsystem_init from percpu_counter_tree_items_size
when the initialization is not already done.
So this probably isn't a way to go?
I'd favor explicit initialization, so the inter-dependencies are clear.
Hmm perhaps we should treat init_mm as a special case in
mm_cpus_allowed() and mm_cpumask().
I'd prefer not to go there if boot sequence permits and keep things
simple.
I think we're in a situation very similar to tree RCU, here is what
is done in rcu_init_geometry:
static bool initialized;
if (initialized) {
/*
* Warn if setup_nr_cpu_ids() had not yet been invoked,
* unless nr_cpus_ids == NR_CPUS, in which case who cares?
*/
WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids);
return;
}
old_nr_cpu_ids = nr_cpu_ids;
initialized = true;
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
