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cgroup: Print message when /proc/cgroups is read on v2-only system

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As a followup to commits 6c2920926b ("cgroup: replace
unified-hierarchy.txt with a proper cgroup v2 documentation") and
ab03125268 ("cgroup: Show # of subsystem CSSes in cgroup.stat"),
add a runtime message to users who read status of controllers in
/proc/cgroups on v2-only system. The detection is based on a)
no controllers are attached to v1, b) default hierarchy is mounted (the
latter is for setups that never mount v2 but read /proc/cgroups upon
boot when controllers default to v2, so that this code may be backported
to older kernels).

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Michal Koutný 2025-03-11 13:36:21 +01:00 • committed by Tejun Heo
parent 77bbb259db
commit a0ab145322
3 changed files with 9 additions and 1 deletions
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1
kernel/cgroup/cgroup-internal.h
View file

@ -168,6 +168,7 @@ struct cgroup_mgctx {
extern struct cgroup_subsys *cgroup_subsys[];
extern struct list_head cgroup_roots;
extern bool cgrp_dfl_visible;
/* iterate across the hierarchies */
#define for_each_root(root) \

7
kernel/cgroup/cgroup-v1.c
View file

@ -673,6 +673,7 @@ struct cftype cgroup1_base_files[] = {
int proc_cgroupstats_show(struct seq_file *m, void *v)
{
struct cgroup_subsys *ss;
bool cgrp_v1_visible = false;
int i;
seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
@ -684,12 +685,18 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
for_each_subsys(ss, i) {
if (cgroup1_subsys_absent(ss))
continue;
cgrp_v1_visible |= ss->root != &cgrp_dfl_root;
seq_printf(m, "%s\t%d\t%d\t%d\n",
ss->legacy_name, ss->root->hierarchy_id,
atomic_read(&ss->root->nr_cgrps),
cgroup_ssid_enabled(i));
}
if (cgrp_dfl_visible && !cgrp_v1_visible)
pr_info_once("/proc/cgroups lists only v1 controllers, use cgroup.controllers of root cgroup for v2 info\n");
return 0;
}

2
kernel/cgroup/cgroup.c
View file

@ -171,7 +171,7 @@ EXPORT_SYMBOL_GPL(cgrp_dfl_root);
* The default hierarchy always exists but is hidden until mounted for the
* first time. This is for backward compatibility.
*/
static bool cgrp_dfl_visible;
bool cgrp_dfl_visible;
/* some controllers are not supported in the default hierarchy */
static u16 cgrp_dfl_inhibit_ss_mask;