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Revert "scx_flatcg: Keep cgroup rb nodes stashed" #375

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Jun 21, 2024
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Revert "scx_flatcg: Keep cgroup rb nodes stashed" #375

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26 changes: 18 additions & 8 deletions scheds/c/scx_flatcg.bpf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -99,7 +99,6 @@ struct cgv_node {
struct bpf_rb_node rb_node;
__u64 cvtime;
__u64 cgid;
struct bpf_refcount refcount;
};

private(CGV_TREE) struct bpf_spin_lock cgv_tree_lock;
Expand Down Expand Up @@ -289,17 +288,14 @@ static void cgrp_enqueued(struct cgroup *cgrp, struct fcg_cgrp_ctx *cgc)
}

stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
if (!stash || !stash->node) {
if (!stash) {
scx_bpf_error("cgv_node lookup failed for cgid %llu", cgid);
return;
}

cgv_node = bpf_refcount_acquire(stash->node);
/* NULL if the node is already on the rbtree */
cgv_node = bpf_kptr_xchg(&stash->node, NULL);
if (!cgv_node) {
/*
* Node never leaves cgv_node_stash, this should only happen if
* fcg_cgroup_exit deletes the stashed node
*/
stat_inc(FCG_STAT_ENQ_RACE);
return;
}
Expand Down Expand Up @@ -612,6 +608,7 @@ void BPF_STRUCT_OPS(fcg_cgroup_set_weight, struct cgroup *cgrp, u32 weight)
static bool try_pick_next_cgroup(u64 *cgidp)
{
struct bpf_rb_node *rb_node;
struct cgv_node_stash *stash;
struct cgv_node *cgv_node;
struct fcg_cgrp_ctx *cgc;
struct cgroup *cgrp;
Expand Down Expand Up @@ -695,6 +692,12 @@ static bool try_pick_next_cgroup(u64 *cgidp)
return true;

out_stash:
stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
if (!stash) {
stat_inc(FCG_STAT_PNC_GONE);
goto out_free;
}

/*
* Paired with cmpxchg in cgrp_enqueued(). If they see the following
* transition, they'll enqueue the cgroup. If they are earlier, we'll
Expand All @@ -707,9 +710,16 @@ static bool try_pick_next_cgroup(u64 *cgidp)
bpf_rbtree_add(&cgv_tree, &cgv_node->rb_node, cgv_node_less);
bpf_spin_unlock(&cgv_tree_lock);
stat_inc(FCG_STAT_PNC_RACE);
return false;
} else {
cgv_node = bpf_kptr_xchg(&stash->node, cgv_node);
if (cgv_node) {
scx_bpf_error("unexpected !NULL cgv_node stash");
goto out_free;
}
}

return false;

out_free:
bpf_obj_drop(cgv_node);
return false;
Expand Down