cgroup_pids: don't account for the root cgroup

Because accounting resources for the root cgroup sometimes incurs
measureable overhead for workloads which don't care about cgroup and
often ends up calculating a number which is available elsewhere in a
slightly different form, cgroup is not in the business of providing
system-wide statistics.  The pids controller which was introduced
recently was exposing "pids.current" at the root.  This patch disable
accounting for root cgroup and removes the file from the root
directory.

While this is a userland visible behavior change, pids has been
available only in one version and was badly broken there, so I don't
think this will be noticeable.  If it turns out to be a problem, we
can reinstate it for v1 hierarchies.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>

diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
index 8e27fc5dbb20e6459a0ff7ab562d6b0f67dbbe27..b50d5a167fda7d2ae6902efd55e551efed780007 100644 (file)
--- a/kernel/cgroup_pids.c
+++ b/kernel/cgroup_pids.c
@@ -106,7 +106,7 @@ static void pids_uncharge(struct pids_cgroup *pids, int num)
 {
        struct pids_cgroup *p;
 
-       for (p = pids; p; p = parent_pids(p))
+       for (p = pids; parent_pids(p); p = parent_pids(p))
                pids_cancel(p, num);
 }
 
@@ -123,7 +123,7 @@ static void pids_charge(struct pids_cgroup *pids, int num)
 {
        struct pids_cgroup *p;
 
-       for (p = pids; p; p = parent_pids(p))
+       for (p = pids; parent_pids(p); p = parent_pids(p))
                atomic64_add(num, &p->counter);
 }
 
@@ -140,7 +140,7 @@ static int pids_try_charge(struct pids_cgroup *pids, int num)
 {
        struct pids_cgroup *p, *q;
 
-       for (p = pids; p; p = parent_pids(p)) {
+       for (p = pids; parent_pids(p); p = parent_pids(p)) {
                int64_t new = atomic64_add_return(num, &p->counter);
 
                /*
@@ -298,6 +298,7 @@ static struct cftype pids_files[] = {
        {
                .name = "current",
                .read_s64 = pids_current_read,
+               .flags = CFTYPE_NOT_ON_ROOT,
        },
        { }     /* terminate */
 };