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Date: Thu, 28 Feb 2019 19:56:41 +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 11:40 AM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> On Thu, Feb 28, 2019 at 11:02 AM Song Liu <songliubraving@...com> wrote:
>>
>>
>>
>>> 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.
>
> Ok, will do.
>
>>
>>>
>>>>
>>>> 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+?
>
> Tbh, adding extra arbitrary constant and emitting warning seems
> excessive. If it's huge number, allocation will fail and we'll return
> error. If it's just big, RSS will be high, and if user cares he should
> be able to deduce that he specified unreasonable size. In general, I
> doubt this value will be overriden often, most uses will probably just
> use default setting.
Fair enough. Let's go with 2G as-is then.
>
>>
>> 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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