[PATCH v2 cgroup/for-5.20] cgroup: Replace cgroup->ancestor_ids[] with ->ancestors[]

Tejun Heo
2022-07-29T20:58:22Z

Every cgroup knows all its ancestors through its ->ancestor_ids[]. There's
no advantage to remembering the IDs instead of the pointers directly and
this makes the array useless for finding an actual ancestor cgroup forcing
cgroup_ancestor() to iteratively walk up the hierarchy instead. Let's
replace cgroup->ancestor_ids[] with ->ancestors[] and remove the walking-up
from cgroup_ancestor().

This patch shouldn't cause user-visible behavior differences.

v2: Update cgroup_ancestor() to use ->ancestors[].

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Acked-by: Namhyung Kim <namhyung-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
---
include/linux/cgroup-defs.h | 10 +++++-----
include/linux/cgroup.h | 8 +++-----
kernel/cgroup/cgroup.c | 7 +++----
net/netfilter/nft_socket.c | 9 +++++----
tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 2 +-
5 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 63bf43c7ca3b..51fa744c2e9d 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -379,7 +379,7 @@ struct cgroup {
/*
* The depth this cgroup is at. The root is at depth zero and each
* step down the hierarchy increments the level. This along with
- * ancestor_ids[] can determine whether a given cgroup is a
+ * ancestors[] can determine whether a given cgroup is a
* descendant of another without traversing the hierarchy.
*/
int level;
@@ -499,8 +499,8 @@ struct cgroup {
/* Used to store internal freezer state */
struct cgroup_freezer_state freezer;

- /* ids of the ancestors at each level including self */
- u64 ancestor_ids[];
+ /* All ancestors including self */
+ struct cgroup *ancestors[];
};

/*
@@ -520,8 +520,8 @@ struct cgroup_root {
/* The root cgroup. Root is destroyed on its release. */
struct cgroup cgrp;

- /* for cgrp->ancestor_ids[0] */
- u64 cgrp_ancestor_id_storage;
+ /* for cgrp->ancestors[0] */
+ u64 cgrp_ancestor_storage;

/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
atomic_t nr_cgrps;
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ed53bfe7c46c..4d143729b246 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -574,7 +574,7 @@ static inline bool cgroup_is_descendant(struct cgroup *cgrp,
{
if (cgrp->root != ancestor->root || cgrp->level < ancestor->level)
return false;
- return cgrp->ancestor_ids[ancestor->level] == cgroup_id(ancestor);
+ return cgrp->ancestors[ancestor->level] == ancestor;
}

/**
@@ -591,11 +591,9 @@ static inline bool cgroup_is_descendant(struct cgroup *cgrp,
static inline struct cgroup *cgroup_ancestor(struct cgroup *cgrp,
int ancestor_level)
{
- if (cgrp->level < ancestor_level)
+ if (ancestor_level < 0 || ancestor_level > cgrp->level)
return NULL;
- while (cgrp && cgrp->level > ancestor_level)
- cgrp = cgroup_parent(cgrp);
- return cgrp;
+ return cgrp->ancestors[ancestor_level];
}

/**
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 85fa4c8587a8..ce587fe43dab 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2047,7 +2047,7 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
}
root_cgrp->kn = kernfs_root_to_node(root->kf_root);
WARN_ON_ONCE(cgroup_ino(root_cgrp) != 1);
- root_cgrp->ancestor_ids[0] = cgroup_id(root_cgrp);
+ root_cgrp->ancestors[0] = root_cgrp;

ret = css_populate_dir(&root_cgrp->self);
if (ret)
@@ -5391,8 +5391,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
int ret;

/* allocate the cgroup and its ID, 0 is reserved for the root */
- cgrp = kzalloc(struct_size(cgrp, ancestor_ids, (level + 1)),
- GFP_KERNEL);
+ cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);
if (!cgrp)
return ERR_PTR(-ENOMEM);

@@ -5444,7 +5443,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,

spin_lock_irq(&css_set_lock);
for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
- cgrp->ancestor_ids[tcgrp->level] = cgroup_id(tcgrp);
+ cgrp->ancestors[tcgrp->level] = tcgrp;

if (tcgrp != cgrp) {
tcgrp->nr_descendants++;
diff --git a/net/netfilter/nft_socket.c b/net/netfilter/nft_socket.c
index 05ae5a338b6f..d982a7c22a77 100644
--- a/net/netfilter/nft_socket.c
+++ b/net/netfilter/nft_socket.c
@@ -40,16 +40,17 @@ static noinline bool
nft_sock_get_eval_cgroupv2(u32 *dest, struct sock *sk, const struct nft_pktinfo *pkt, u32 level)
{
struct cgroup *cgrp;
+ u64 cgid;

if (!sk_fullsock(sk))
return false;

- cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
- if (level > cgrp->level)
+ cgrp = cgroup_ancestor(sock_cgroup_ptr(&sk->sk_cgrp_data), level);
+ if (!cgrp)
return false;

- memcpy(dest, &cgrp->ancestor_ids[level], sizeof(u64));
-
+ cgid = cgroup_id(cgrp);
+ memcpy(dest, &cgid, sizeof(u64));
return true;
}
#endif
diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
index 292c430768b5..bd6a420acc8f 100644
--- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
+++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
@@ -68,7 +68,7 @@ static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
break;

// convert cgroup-id to a map index
- cgrp_id = BPF_CORE_READ(cgrp, ancestor_ids[i]);
+ cgrp_id = BPF_CORE_READ(cgrp, ancestors[i], kn, id);
elem = bpf_map_lookup_elem(&cgrp_idx, &cgrp_id);
if (!elem)
continue;