From c5c00b2417a632fb6e456144d4d3fb5a6ede8931 Mon Sep 17 00:00:00 2001
From: Valentin Schneider <valentin.schneider@arm.com>
Date: Tue, 18 May 2021 14:07:25 +0100
Subject: [PATCH] sched/debug: Don't update sched_domain debug directories
before sched_debug_init()
References: git fixes (sched) stable-5.14.4
Patch-mainline: v5.15-rc1
Git-commit: 459b09b5a3254008b63382bf41a9b36d0b590f57
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
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/sched/debug.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 0c5ec2776ddf..7e08e3d947c2 100644
--- 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;