Re: [RFC PATCH v2] bpf: Using binary search to improve the performance of btf_find_by_name_kind
From: Eduard Zingerman
Date: Tue Sep 12 2023 - 10:19:20 EST
On Tue, 2023-09-12 at 16:51 +0300, Eduard Zingerman wrote:
> On Sat, 2023-09-09 at 02:16 -0700, Donglin Peng wrote:
> > Currently, we are only using the linear search method to find the type id
> > by the name, which has a time complexity of O(n). This change involves
> > sorting the names of btf types in ascending order and using binary search,
> > which has a time complexity of O(log(n)). This idea was inspired by the
> > following patch:
> >
> > 60443c88f3a8 ("kallsyms: Improve the performance of kallsyms_lookup_name()").
> >
> > At present, this improvement is only for searching in vmlinux's and
> > module's BTFs, and the kind should only be BTF_KIND_FUNC or BTF_KIND_STRUCT.
> >
> > Another change is the search direction, where we search the BTF first and
> > then its base, the type id of the first matched btf_type will be returned.
> >
> > Here is a time-consuming result that finding all the type ids of 67,819 kernel
> > functions in vmlinux's BTF by their names:
> >
> > Before: 17000 ms
> > After: 10 ms
> >
> > The average lookup performance has improved about 1700x at the above scenario.
> >
> > However, this change will consume more memory, for example, 67,819 kernel
> > functions will allocate about 530KB memory.
>
> Hi Donglin,
>
> I think this is a good improvement. However, I wonder, why did you
> choose to have a separate name map for each BTF kind?
>
> I did some analysis for my local testing kernel config and got such numbers:
> - total number of BTF objects: 97350
> - number of FUNC and STRUCT objects: 51597
> - number of FUNC, STRUCT, UNION, ENUM, ENUM64, TYPEDEF, DATASEC objects: 56817
> (these are all kinds for which lookup by name might make sense)
> - number of named objects: 54246
> - number of name collisions:
> - unique names: 53985 counts
> - 2 objects with the same name: 129 counts
> - 3 objects with the same name: 3 counts
>
> So, it appears that having a single map for all named objects makes
> sense and would also simplify the implementation, what do you think?
Some more numbers for my config:
- 13241 types (struct, union, typedef, enum), log2 13241 = 13.7
- 43575 funcs, log2 43575 = 15.4
Thus, having separate map for types vs functions might save ~1.7
search iterations. Is this a significant slowdown in practice?
>
> Thanks,
> Eduard
>
> >
> > Signed-off-by: Donglin Peng <pengdonglin@xxxxxxxxxxxxxx>
> > ---
> > Changes in RFC v2:
> > - Fix the build issue reported by kernel test robot <lkp@xxxxxxxxx>
> > ---
> > include/linux/btf.h | 1 +
> > kernel/bpf/btf.c | 300 ++++++++++++++++++++++++++++++++++++++++++--
> > 2 files changed, 291 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index cac9f304e27a..6260a0668773 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -201,6 +201,7 @@ bool btf_is_kernel(const struct btf *btf);
> > bool btf_is_module(const struct btf *btf);
> > struct module *btf_try_get_module(const struct btf *btf);
> > u32 btf_nr_types(const struct btf *btf);
> > +u32 btf_type_cnt(const struct btf *btf);
> > bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
> > const struct btf_member *m,
> > u32 expected_offset, u32 expected_size);
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 817204d53372..51aa9f27853b 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -240,6 +240,26 @@ struct btf_id_dtor_kfunc_tab {
> > struct btf_id_dtor_kfunc dtors[];
> > };
> >
> > +enum {
> > + BTF_ID_NAME_FUNC, /* function */
> > + BTF_ID_NAME_STRUCT, /* struct */
> > + BTF_ID_NAME_MAX
> > +};
> > +
> > +struct btf_id_name {
> > + int id;
> > + u32 name_off;
> > +};
> > +
> > +struct btf_id_name_map {
> > + struct btf_id_name *id_name;
> > + u32 count;
> > +};
> > +
> > +struct btf_id_name_maps {
> > + struct btf_id_name_map map[BTF_ID_NAME_MAX];
> > +};
> > +
> > struct btf {
> > void *data;
> > struct btf_type **types;
> > @@ -257,6 +277,7 @@ struct btf {
> > struct btf_kfunc_set_tab *kfunc_set_tab;
> > struct btf_id_dtor_kfunc_tab *dtor_kfunc_tab;
> > struct btf_struct_metas *struct_meta_tab;
> > + struct btf_id_name_maps *id_name_maps;
> >
> > /* split BTF support */
> > struct btf *base_btf;
> > @@ -532,22 +553,142 @@ u32 btf_nr_types(const struct btf *btf)
> > return total;
> > }
> >
> > +u32 btf_type_cnt(const struct btf *btf)
> > +{
> > + return btf->start_id + btf->nr_types;
> > +}
> > +
> > +static inline u8 btf_id_name_idx_to_kind(int index)
> > +{
> > + u8 kind;
> > +
> > + switch (index) {
> > + case BTF_ID_NAME_FUNC:
> > + kind = BTF_KIND_FUNC;
> > + break;
> > + case BTF_ID_NAME_STRUCT:
> > + kind = BTF_KIND_STRUCT;
> > + break;
> > + default:
> > + kind = BTF_KIND_UNKN;
> > + break;
> > + }
> > +
> > + return kind;
> > +}
> > +
> > +static inline int btf_id_name_kind_to_idx(u8 kind)
> > +{
> > + int index;
> > +
> > + switch (kind) {
> > + case BTF_KIND_FUNC:
> > + index = BTF_ID_NAME_FUNC;
> > + break;
> > + case BTF_KIND_STRUCT:
> > + index = BTF_ID_NAME_STRUCT;
> > + break;
> > + default:
> > + index = -1;
> > + break;
> > + }
> > +
> > + return index;
> > +}
> > +
> > +static s32 btf_find_by_name_bsearch(struct btf_id_name *id_name,
> > + u32 size, const char *name,
> > + struct btf_id_name **start,
> > + struct btf_id_name **end,
> > + const struct btf *btf)
> > +{
> > + int ret;
> > + int low, mid, high;
> > + const char *name_buf;
> > +
> > + low = 0;
> > + high = size - 1;
> > +
> > + while (low <= high) {
> > + mid = low + (high - low) / 2;
> > + name_buf = btf_name_by_offset(btf, id_name[mid].name_off);
> > + ret = strcmp(name, name_buf);
> > + if (ret > 0)
> > + low = mid + 1;
> > + else if (ret < 0)
> > + high = mid - 1;
> > + else
> > + break;
> > + }
> > +
> > + if (low > high)
> > + return -ESRCH;
> > +
> > + if (start) {
> > + low = mid;
> > + while (low) {
> > + name_buf = btf_name_by_offset(btf, id_name[low-1].name_off);
> > + if (strcmp(name, name_buf))
> > + break;
> > + low--;
> > + }
> > + *start = &id_name[low];
> > + }
> > +
> > + if (end) {
> > + high = mid;
> > + while (high < size - 1) {
> > + name_buf = btf_name_by_offset(btf, id_name[high+1].name_off);
> > + if (strcmp(name, name_buf))
> > + break;
> > + high++;
> > + }
> > + *end = &id_name[high];
> > + }
> > +
> > + return id_name[mid].id;
> > +}
> > +
> > s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
> > {
> > + const struct btf_id_name_maps *maps;
> > + const struct btf_id_name_map *map;
> > + struct btf_id_name *start;
> > const struct btf_type *t;
> > const char *tname;
> > - u32 i, total;
> > + int index = btf_id_name_kind_to_idx(kind);
> > + s32 id, total;
> >
> > - total = btf_nr_types(btf);
> > - for (i = 1; i < total; i++) {
> > - t = btf_type_by_id(btf, i);
> > - if (BTF_INFO_KIND(t->info) != kind)
> > - continue;
> > + do {
> > + maps = btf->id_name_maps;
> > + if (index >= 0 && maps && maps->map[index].id_name) {
> > + /* binary search */
> > + map = &maps->map[index];
> > + id = btf_find_by_name_bsearch(map->id_name,
> > + map->count, name, &start, NULL, btf);
> > + if (id > 0) {
> > + /*
> > + * Return the first one that
> > + * matched
> > + */
> > + return start->id;
> > + }
> > + } else {
> > + /* linear search */
> > + total = btf_type_cnt(btf);
> > + for (id = btf->start_id; id < total; id++) {
> > + t = btf_type_by_id(btf, id);
> > + if (BTF_INFO_KIND(t->info) != kind)
> > + continue;
> > +
> > + tname = btf_name_by_offset(btf, t->name_off);
> > + if (!strcmp(tname, name))
> > + return id;
> > + }
> > + }
> >
> > - tname = btf_name_by_offset(btf, t->name_off);
> > - if (!strcmp(tname, name))
> > - return i;
> > - }
> > + btf = btf->base_btf;
> > + } while (btf);
> >
> > return -ENOENT;
> > }
> > @@ -1639,6 +1780,32 @@ static void btf_free_id(struct btf *btf)
> > spin_unlock_irqrestore(&btf_idr_lock, flags);
> > }
> >
> > +static void btf_destroy_id_name(struct btf *btf, int index)
> > +{
> > + struct btf_id_name_maps *maps = btf->id_name_maps;
> > + struct btf_id_name_map *map = &maps->map[index];
> > +
> > + if (map->id_name) {
> > + kvfree(map->id_name);
> > + map->id_name = NULL;
> > + map->count = 0;
> > + }
> > +}
> > +
> > +static void btf_destroy_id_name_map(struct btf *btf)
> > +{
> > + int i;
> > +
> > + if (!btf->id_name_maps)
> > + return;
> > +
> > + for (i = 0; i < BTF_ID_NAME_MAX; i++)
> > + btf_destroy_id_name(btf, i);
> > +
> > + kfree(btf->id_name_maps);
> > + btf->id_name_maps = NULL;
> > +}
> > +
> > static void btf_free_kfunc_set_tab(struct btf *btf)
> > {
> > struct btf_kfunc_set_tab *tab = btf->kfunc_set_tab;
> > @@ -1689,6 +1856,7 @@ static void btf_free_struct_meta_tab(struct btf *btf)
> >
> > static void btf_free(struct btf *btf)
> > {
> > + btf_destroy_id_name_map(btf);
> > btf_free_struct_meta_tab(btf);
> > btf_free_dtor_kfunc_tab(btf);
> > btf_free_kfunc_set_tab(btf);
> > @@ -5713,6 +5881,107 @@ int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_ty
> > return kctx_type_id;
> > }
> >
> > +static int btf_compare_id_name(const void *a, const void *b, const void *priv)
> > +{
> > + const struct btf_id_name *ia = (const struct btf_id_name *)a;
> > + const struct btf_id_name *ib = (const struct btf_id_name *)b;
> > + const struct btf *btf = priv;
> > + int ret;
> > +
> > + /*
> > + * Sort names in ascending order, if the name is same, sort ids in
> > + * ascending order.
> > + */
> > + ret = strcmp(btf_name_by_offset(btf, ia->name_off),
> > + btf_name_by_offset(btf, ib->name_off));
> > + if (!ret)
> > + ret = ia->id - ib->id;
> > +
> > + return ret;
> > +}
> > +
> > +static int btf_create_id_name(struct btf *btf, int index)
> > +{
> > + struct btf_id_name_maps *maps = btf->id_name_maps;
> > + struct btf_id_name_map *map = &maps->map[index];
> > + const struct btf_type *t;
> > + struct btf_id_name *id_name;
> > + const char *name;
> > + int i, j = 0;
> > + u32 total, count = 0;
> > + u8 kind;
> > +
> > + kind = btf_id_name_idx_to_kind(index);
> > + if (kind == BTF_KIND_UNKN)
> > + return -EINVAL;
> > +
> > + if (map->id_name || map->count != 0)
> > + return -EINVAL;
> > +
> > + total = btf_type_cnt(btf);
> > + for (i = btf->start_id; i < total; i++) {
> > + t = btf_type_by_id(btf, i);
> > + if (BTF_INFO_KIND(t->info) != kind)
> > + continue;
> > + name = btf_name_by_offset(btf, t->name_off);
> > + if (str_is_empty(name))
> > + continue;
> > + count++;
> > + }
> > +
> > + if (count == 0)
> > + return 0;
> > +
> > + id_name = kvcalloc(count, sizeof(struct btf_id_name),
> > + GFP_KERNEL);
> > + if (!id_name)
> > + return -ENOMEM;
> > +
> > + for (i = btf->start_id; i < total; i++) {
> > + t = btf_type_by_id(btf, i);
> > + if (BTF_INFO_KIND(t->info) != kind)
> > + continue;
> > + name = btf_name_by_offset(btf, t->name_off);
> > + if (str_is_empty(name))
> > + continue;
> > +
> > + id_name[j].id = i;
> > + id_name[j].name_off = t->name_off;
> > + j++;
> > + }
> > +
> > + sort_r(id_name, count, sizeof(id_name[0]), btf_compare_id_name,
> > + NULL, btf);
> > +
> > + map->id_name = id_name;
> > + map->count = count;
> > +
> > + return 0;
> > +}
> > +
> > +static int btf_create_id_name_map(struct btf *btf)
> > +{
> > + int err, i;
> > + struct btf_id_name_maps *maps;
> > +
> > + if (btf->id_name_maps)
> > + return -EBUSY;
> > +
> > + maps = kzalloc(sizeof(struct btf_id_name_maps), GFP_KERNEL);
> > + if (!maps)
> > + return -ENOMEM;
> > +
> > + btf->id_name_maps = maps;
> > +
> > + for (i = 0; i < BTF_ID_NAME_MAX; i++) {
> > + err = btf_create_id_name(btf, i);
> > + if (err < 0)
> > + break;
> > + }
> > +
> > + return err;
> > +}
> > +
> > BTF_ID_LIST(bpf_ctx_convert_btf_id)
> > BTF_ID(struct, bpf_ctx_convert)
> >
> > @@ -5760,6 +6029,10 @@ struct btf *btf_parse_vmlinux(void)
> > if (err)
> > goto errout;
> >
> > + err = btf_create_id_name_map(btf);
> > + if (err)
> > + goto errout;
> > +
> > /* btf_parse_vmlinux() runs under bpf_verifier_lock */
> > bpf_ctx_convert.t = btf_type_by_id(btf, bpf_ctx_convert_btf_id[0]);
> >
> > @@ -5777,6 +6050,7 @@ struct btf *btf_parse_vmlinux(void)
> > errout:
> > btf_verifier_env_free(env);
> > if (btf) {
> > + btf_destroy_id_name_map(btf);
> > kvfree(btf->types);
> > kfree(btf);
> > }
> > @@ -5844,13 +6118,19 @@ static struct btf *btf_parse_module(const char *module_name, const void *data, u
> > if (err)
> > goto errout;
> >
> > + err = btf_create_id_name_map(btf);
> > + if (err)
> > + goto errout;
> > +
> > btf_verifier_env_free(env);
> > refcount_set(&btf->refcnt, 1);
> > +
> > return btf;
> >
> > errout:
> > btf_verifier_env_free(env);
> > if (btf) {
> > + btf_destroy_id_name_map(btf);
> > kvfree(btf->data);
> > kvfree(btf->types);
> > kfree(btf);
>