netdev - [PATCH bpf] bpf: Improve bucket

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Date:   Fri, 7 Feb 2020 00:18:10 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     <bpf@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>, <kernel-team@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux-Sparse <linux-sparse@...r.kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        <netdev@...r.kernel.org>, Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH bpf] bpf: Improve bucket_log calculation logic

It was reported that the max_t, ilog2, and roundup_pow_of_two macros have
exponential effects on the number of states in the sparse checker.

This patch breaks them up by calculating the "nbuckets" first so
that the "bucket_log" only needs to take ilog2().

Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
Reported-by: Randy Dunlap <rdunlap@...radead.org>
Reported-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Martin KaFai Lau <kafai@...com>
---
 net/core/bpf_sk_storage.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index 458be6b3eda9..3ab23f698221 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -643,9 +643,10 @@ static struct bpf_map *bpf_sk_storage_map_alloc(union bpf_attr *attr)
 		return ERR_PTR(-ENOMEM);
 	bpf_map_init_from_attr(&smap->map, attr);
 
+	nbuckets = roundup_pow_of_two(num_possible_cpus());
 	/* Use at least 2 buckets, select_bucket() is undefined behavior with 1 bucket */
-	smap->bucket_log = max_t(u32, 1, ilog2(roundup_pow_of_two(num_possible_cpus())));
-	nbuckets = 1U << smap->bucket_log;
+	nbuckets = max_t(u32, 2, nbuckets);
+	smap->bucket_log = ilog2(nbuckets);
 	cost = sizeof(*smap->buckets) * nbuckets + sizeof(*smap);
 
 	ret = bpf_map_charge_init(&smap->map.memory, cost);
-- 
2.17.1