Re: [RFC PATCH v2] bpf: Using binary search to improve the performance of btf_find_by_name_kind

[Eduard Zingerman]

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?

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);