[PATCH net-next 3/3] bpf: Only set node->ref = 1 if it has not been set

[Martin KaFai Lau]

This patch writes 'node->ref = 1' only if node->ref is 0.
The number of lookups/s for a ~1M entries LRU map increased by
~30% (260097 to 343313).

Other writes on 'node->ref = 0' is not changed.  In those cases, the
same cache line has to be changed anyway.

First column: Size of the LRU hash
Second column: Number of lookups/s

Before:
> echo "$((2**20+1)): $(./map_perf_test 1024 1 $((2**20+1)) 10000000 | awk 
> '{print $3}')"
1048577: 260097

After:
> echo "$((2**20+1)): $(./map_perf_test 1024 1 $((2**20+1)) 10000000 | awk 
> '{print $3}')"
1048577: 343313

Signed-off-by: Martin KaFai Lau <ka...@fb.com>
---
 kernel/bpf/bpf_lru_list.h | 3 ++-
 kernel/bpf/hashtab.c      | 7 ++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/bpf_lru_list.h b/kernel/bpf/bpf_lru_list.h
index 5c35a98d02bf..7d4f89b7cb84 100644
--- a/kernel/bpf/bpf_lru_list.h
+++ b/kernel/bpf/bpf_lru_list.h
@@ -69,7 +69,8 @@ static inline void bpf_lru_node_set_ref(struct bpf_lru_node 
*node)
        /* ref is an approximation on access frequency.  It does not
         * have to be very accurate.  Hence, no protection is used.
         */
-       node->ref = 1;
+       if (!node->ref)
+               node->ref = 1;
 }
 
 int bpf_lru_init(struct bpf_lru *lru, bool percpu, u32 hash_offset,
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 682f4543fefa..431126f31ea3 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -519,9 +519,14 @@ static u32 htab_lru_map_gen_lookup(struct bpf_map *map,
 {
        struct bpf_insn *insn = insn_buf;
        const int ret = BPF_REG_0;
+       const int ref_reg = BPF_REG_1;
 
        *insn++ = BPF_EMIT_CALL((u64 (*)(u64, u64, u64, u64, 
u64))__htab_map_lookup_elem);
-       *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 2);
+       *insn++ = BPF_JMP_IMM(BPF_JEQ, ret, 0, 4);
+       *insn++ = BPF_LDX_MEM(BPF_B, ref_reg, ret,
+                             offsetof(struct htab_elem, lru_node) +
+                             offsetof(struct bpf_lru_node, ref));
+       *insn++ = BPF_JMP_IMM(BPF_JNE, ref_reg, 0, 1);
        *insn++ = BPF_ST_MEM(BPF_B, ret,
                             offsetof(struct htab_elem, lru_node) +
                             offsetof(struct bpf_lru_node, ref),
-- 
2.9.5