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Date: Thu, 28 Feb 2019 19:02:21 +0000
From: Song Liu <songliubraving@...com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
CC: Song Liu <liu.song.a23@...il.com>,
Andrii Nakryiko <andriin@...com>,
Kernel Team <Kernel-team@...com>,
Alexei Starovoitov <ast@...com>,
"Arnaldo Carvalho de Melo" <acme@...nel.org>,
Networking <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 3/5] btf: allow to customize dedup hash table
size
> On Feb 28, 2019, at 10:51 AM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> On Thu, Feb 28, 2019 at 10:27 AM Song Liu <liu.song.a23@...il.com> wrote:
>>
>> On Wed, Feb 27, 2019 at 2:47 PM Andrii Nakryiko <andriin@...com> wrote:
>>>
>>> Default size of dedup table (16k) is good enough for most binaries, even
>>> typical vmlinux images. But there are cases of binaries with huge amount
>>> of BTF types (e.g., allyesconfig variants of kernel), which benefit from
>>> having bigger dedup table size to lower amount of unnecessary hash
>>> collisions. Tools like pahole, thus, can tune this parameter to reach
>>> optimal performance.
>>>
>>> This change also serves double purpose of allowing tests to force hash
>>> collisions to test some corner cases, used in follow up patch.
>>>
>>> Signed-off-by: Andrii Nakryiko <andriin@...com>
>>> ---
>>> tools/lib/bpf/btf.c | 43 ++++++++++++++++++++++++++-----------------
>>> tools/lib/bpf/btf.h | 1 +
>>> 2 files changed, 27 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>>> index 68b50e9bbde1..6bbb710216e6 100644
>>> --- a/tools/lib/bpf/btf.c
>>> +++ b/tools/lib/bpf/btf.c
>>> @@ -1070,8 +1070,7 @@ int btf__dedup(struct btf *btf, struct btf_ext *btf_ext,
>>> return err;
>>> }
>>>
>>> -#define BTF_DEDUP_TABLE_SIZE_LOG 14
>>> -#define BTF_DEDUP_TABLE_MOD ((1 << BTF_DEDUP_TABLE_SIZE_LOG) - 1)
>>> +#define BTF_DEDUP_TABLE_DEFAULT_SIZE (1 << 14)
>>> #define BTF_UNPROCESSED_ID ((__u32)-1)
>>> #define BTF_IN_PROGRESS_ID ((__u32)-2)
>>>
>>> @@ -1128,18 +1127,21 @@ static inline __u32 hash_combine(__u32 h, __u32 value)
>>> #undef GOLDEN_RATIO_PRIME
>>> }
>>>
>>> -#define for_each_hash_node(table, hash, node) \
>>> - for (node = table[hash & BTF_DEDUP_TABLE_MOD]; node; node = node->next)
>>> +#define for_each_dedup_cand(d, hash, node) \
>>> + for (node = d->dedup_table[hash & (d->opts.dedup_table_size - 1)]; \
>>> + node; \
>>> + node = node->next)
>>>
>>> static int btf_dedup_table_add(struct btf_dedup *d, __u32 hash, __u32 type_id)
>>> {
>>> struct btf_dedup_node *node = malloc(sizeof(struct btf_dedup_node));
>>> + int bucket = hash & (d->opts.dedup_table_size - 1);
>>>
>>> if (!node)
>>> return -ENOMEM;
>>> node->type_id = type_id;
>>> - node->next = d->dedup_table[hash & BTF_DEDUP_TABLE_MOD];
>>> - d->dedup_table[hash & BTF_DEDUP_TABLE_MOD] = node;
>>> + node->next = d->dedup_table[bucket];
>>> + d->dedup_table[bucket] = node;
>>> return 0;
>>> }
>>>
>>> @@ -1177,7 +1179,7 @@ static void btf_dedup_table_free(struct btf_dedup *d)
>>> if (!d->dedup_table)
>>> return;
>>>
>>> - for (i = 0; i < (1 << BTF_DEDUP_TABLE_SIZE_LOG); i++) {
>>> + for (i = 0; i < d->opts.dedup_table_size; i++) {
>>> while (d->dedup_table[i]) {
>>> tmp = d->dedup_table[i];
>>> d->dedup_table[i] = tmp->next;
>>> @@ -1221,10 +1223,19 @@ static struct btf_dedup *btf_dedup_new(struct btf *btf, struct btf_ext *btf_ext,
>>> if (!d)
>>> return ERR_PTR(-ENOMEM);
>>>
>>> + d->opts.dont_resolve_fwds = opts && opts->dont_resolve_fwds;
>>> + /* ensure table size is power of two and limit to 2G */
>>> + d->opts.dedup_table_size = opts && opts->dedup_table_size
>>> + ? opts->dedup_table_size
>>> + : BTF_DEDUP_TABLE_DEFAULT_SIZE;
>>> + for (i = 0; i < 31 && (1 << i) < d->opts.dedup_table_size; i++)
>>> + ;
>>> + d->opts.dedup_table_size = 1 << i;
>>> +
>> So this is the roundup log2 logic? How about we define some marcos
>> to make it cleaner? Like
>>
>> #define BTF_DEDUP_TABLE_MAX_SIZE xxxx
>
> You mean hide this loop behind macro? Or specify max size as a macro?
> If former, I'd rather do static function, something like
> roundup_pow_of_2_with_max?
I meant specify max size as a macro. Doing static function is also a
good idea.
>
>>
>> Also, how big hash table do we need for allyesconfig? 2G seems really
>> big to me.
>
> It works even with 16k and takes about 45 seconds. I didn't want to
> artificially limit this to something small and left it up to user.
> This algorithm can be used for arbitrary binaries after pahole's
> dwarf2btf conversion, which could be much bigger than kernel, so I
> didn't want to artificially limit this. Realistically, unlikely that
> you'll need more than 64k-128k.
How about we show some warning like "You are using really big size" for
too big numbers, like 16M+?
Thanks,
Song
>
>>
>>> d->btf = btf;
>>> d->btf_ext = btf_ext;
>>>
>>> - d->dedup_table = calloc(1 << BTF_DEDUP_TABLE_SIZE_LOG,
>>> + d->dedup_table = calloc(d->opts.dedup_table_size,
>>> sizeof(struct btf_dedup_node *));
>>> if (!d->dedup_table) {
>>> err = -ENOMEM;
>>> @@ -1249,8 +1260,6 @@ static struct btf_dedup *btf_dedup_new(struct btf *btf, struct btf_ext *btf_ext,
>>> for (i = 0; i <= btf->nr_types; i++)
>>> d->hypot_map[i] = BTF_UNPROCESSED_ID;
>>>
>>> - d->opts.dont_resolve_fwds = opts && opts->dont_resolve_fwds;
>>> -
>>> done:
>>> if (err) {
>>> btf_dedup_free(d);
>>> @@ -1824,7 +1833,7 @@ static int btf_dedup_prim_type(struct btf_dedup *d, __u32 type_id)
>>>
>>> case BTF_KIND_INT:
>>> h = btf_hash_int(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_int(t, cand)) {
>>> new_id = cand_node->type_id;
>>> @@ -1835,7 +1844,7 @@ static int btf_dedup_prim_type(struct btf_dedup *d, __u32 type_id)
>>>
>>> case BTF_KIND_ENUM:
>>> h = btf_hash_enum(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_enum(t, cand)) {
>>> new_id = cand_node->type_id;
>>> @@ -1846,7 +1855,7 @@ static int btf_dedup_prim_type(struct btf_dedup *d, __u32 type_id)
>>>
>>> case BTF_KIND_FWD:
>>> h = btf_hash_common(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_common(t, cand)) {
>>> new_id = cand_node->type_id;
>>> @@ -2263,7 +2272,7 @@ static int btf_dedup_struct_type(struct btf_dedup *d, __u32 type_id)
>>> return 0;
>>>
>>> h = btf_hash_struct(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> int eq;
>>>
>>> btf_dedup_clear_hypot_map(d);
>>> @@ -2349,7 +2358,7 @@ static int btf_dedup_ref_type(struct btf_dedup *d, __u32 type_id)
>>> t->type = ref_type_id;
>>>
>>> h = btf_hash_common(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_common(t, cand)) {
>>> new_id = cand_node->type_id;
>>> @@ -2372,7 +2381,7 @@ static int btf_dedup_ref_type(struct btf_dedup *d, __u32 type_id)
>>> info->index_type = ref_type_id;
>>>
>>> h = btf_hash_array(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_array(t, cand)) {
>>> new_id = cand_node->type_id;
>>> @@ -2403,7 +2412,7 @@ static int btf_dedup_ref_type(struct btf_dedup *d, __u32 type_id)
>>> }
>>>
>>> h = btf_hash_fnproto(t);
>>> - for_each_hash_node(d->dedup_table, h, cand_node) {
>>> + for_each_dedup_cand(d, h, cand_node) {
>>> cand = d->btf->types[cand_node->type_id];
>>> if (btf_equal_fnproto(t, cand)) {
>>> new_id = cand_node->type_id;
>>> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
>>> index b60bb7cf5fff..28a1e1e59861 100644
>>> --- a/tools/lib/bpf/btf.h
>>> +++ b/tools/lib/bpf/btf.h
>>> @@ -90,6 +90,7 @@ LIBBPF_API __u32 btf_ext__func_info_rec_size(const struct btf_ext *btf_ext);
>>> LIBBPF_API __u32 btf_ext__line_info_rec_size(const struct btf_ext *btf_ext);
>>>
>>> struct btf_dedup_opts {
>>> + unsigned int dedup_table_size;
>>> bool dont_resolve_fwds;
>>> };
>>>
>>> --
>>> 2.17.1
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