sched/debug: Don't update sched_domain debug directories before sched_debug_init()

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: 3b87f136f8fc ("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

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 0c5ec27..7e08e3d 100644 (file)
--- a/kernel/sched/debug.c
+++ b/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;