sched/debug: Don't update sched_domain debug directories before sched…

…_debug_init()

[ Upstream commit 459b09b ]

Since CPU capacity asymmetry can stem purely from maximum frequency
differences (e.g. Pixel 1), a rebuild of the scheduler topology can be
issued upon loading cpufreq, see:

  arch_topology.c::init_cpu_capacity_callback()

Turns out that if this rebuild happens *before* sched_debug_init() is
run (which is a late initcall), we end up messing up the sched_domain debug
directory: passing a NULL parent to debugfs_create_dir() ends up creating
the directory at the debugfs root, which in this case creates
/sys/kernel/debug/domains (instead of /sys/kernel/debug/sched/domains).

This currently doesn't happen on asymmetric systems which use cpufreq-scpi
or cpufreq-dt drivers, as those are loaded via
deferred_probe_initcall() (it is also a late initcall, but appears to be
ordered *after* sched_debug_init()).

Ionela has been working on detecting maximum frequency asymmetry via ACPI,
and that actually happens via a *device* initcall, thus before
sched_debug_init(), and causes the aforementionned debugfs mayhem.

One option would be to punt sched_debug_init() down to
fs_initcall_sync(). Preventing update_sched_domain_debugfs() from running
before sched_debug_init() appears to be the safer option.

Fixes: 3b87f13 ("sched,debug: Convert sysctl sched_domains to debugfs")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lore.kernel.org/r/20210514095339.12979-1-ionela.voinescu@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/sched/debug.c

@@ -388,6 +388,13 @@ void update_sched_domain_debugfs(void)
 {
 	int cpu, i;
 
+	/*
+	 * This can unfortunately be invoked before sched_debug_init() creates
+	 * the debug directory. Don't touch sd_sysctl_cpus until then.
+	 */
+	if (!debugfs_sched)
+		return;
+
 	if (!cpumask_available(sd_sysctl_cpus)) {
 		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
 			return;