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    Subject[PATCH AUTOSEL 5.5 136/542] bpf: Improve bucket_log calculation logic
    Date
    From: Martin KaFai Lau <kafai@fb.com>

    [ Upstream commit 88d6f130e5632bbf419a2e184ec7adcbe241260b ]

    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().

    In addition, Linus mentioned:

    Patch looks good, but I'd like to point out that it's not just sparse.

    You can see it with a simple

    make net/core/bpf_sk_storage.i
    grep 'smap->bucket_log = ' net/core/bpf_sk_storage.i | wc

    and see the end result:

    1 365071 2686974

    That's one line (the assignment line) that is 2,686,974 characters in
    length.

    Now, sparse does happen to react particularly badly to that (I didn't
    look to why, but I suspect it's just that evaluating all the types
    that don't actually ever end up getting used ends up being much more
    expensive than it should be), but I bet it's not good for gcc either.

    Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
    Reported-by: Randy Dunlap <rdunlap@infradead.org>
    Reported-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
    Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Martin KaFai Lau <kafai@fb.com>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
    Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
    Link: https://lore.kernel.org/bpf/20200207081810.3918919-1-kafai@fb.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 458be6b3eda97..3ab23f698221c 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);
    --
    2.20.1
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