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Message-ID: <CAEf4BzYBffp+47SLaV5sMWVAVSbxyDX3DnNoOju1y9wr+wupeQ@mail.gmail.com>
Date: Tue, 29 Oct 2024 15:13:14 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Donglin Peng <dolinux.peng@...il.com>
Cc: andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, rostedt@...dmis.org,
mhiramat@...nel.org, bpf@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] bpf: Using binary search to improve the
performance of btf_find_by_name_kind
On Mon, Oct 28, 2024 at 5:22 PM Donglin Peng <dolinux.peng@...il.com> 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.
>
> 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 87590 type ids by their names in
> vmlinux's BTF.
>
> Before: 158426 ms
> After: 114 ms
>
> The average lookup performance has improved more than 1000x in the above scenario.
>
> Tested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Signed-off-by: Donglin Peng <dolinux.peng@...il.com>
> ---
> v4:
> - move the modification of libbpf to another patch
>
> v3:
> - Link: https://lore.kernel.org/all/20240608140835.965949-1-dolinux.peng@gmail.com/
> - Sort btf_types during build process other than during boot, to reduce the
> overhead of memory and boot time.
>
> v2:
> - Link: https://lore.kernel.org/all/20230909091646.420163-1-pengdonglin@sangfor.com.cn
> ---
> include/linux/btf.h | 1 +
> kernel/bpf/btf.c | 157 ++++++++++++++++++++++++++++++++++++++++----
> 2 files changed, 147 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index b8a583194c4a..64c35aaa22fa 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -216,6 +216,7 @@ bool btf_is_module(const struct btf *btf);
> bool btf_is_vmlinux(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);
> struct btf *btf_base_btf(const struct btf *btf);
> bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
> const struct btf_member *m,
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 5cd1c7a23848..6d0d58989640 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -262,6 +262,7 @@ struct btf {
> u32 data_size;
> refcount_t refcnt;
> u32 id;
> + u32 nr_types_sorted;
> struct rcu_head rcu;
> struct btf_kfunc_set_tab *kfunc_set_tab;
> struct btf_id_dtor_kfunc_tab *dtor_kfunc_tab;
> @@ -548,23 +549,102 @@ u32 btf_nr_types(const struct btf *btf)
> return total;
> }
>
> -s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
> +u32 btf_type_cnt(const struct btf *btf)
> +{
> + return btf->start_id + btf->nr_types;
> +}
> +
> +static s32 btf_find_by_name_bsearch(const struct btf *btf, const char *name,
> + int *start, int *end)
> {
> const struct btf_type *t;
> - const char *tname;
> - u32 i, total;
> + const char *name_buf;
> + int low, low_start, mid, high, high_end;
> + int ret, start_id;
> +
> + start_id = btf->base_btf ? btf->start_id : 1;
> + low_start = low = start_id;
> + high_end = high = start_id + btf->nr_types_sorted - 1;
> +
> + while (low <= high) {
> + mid = low + (high - low) / 2;
> + t = btf_type_by_id(btf, mid);
> + name_buf = btf_name_by_offset(btf, t->name_off);
> + ret = strcmp(name, name_buf);
> + if (ret > 0)
> + low = mid + 1;
> + else if (ret < 0)
> + high = mid - 1;
> + else
> + break;
> + }
>
> - 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;
> + if (low > high)
> + return -ESRCH;
>
> - tname = btf_name_by_offset(btf, t->name_off);
> - if (!strcmp(tname, name))
> - return i;
> + if (start) {
> + low = mid;
> + while (low > low_start) {
> + t = btf_type_by_id(btf, low-1);
> + name_buf = btf_name_by_offset(btf, t->name_off);
> + if (strcmp(name, name_buf))
> + break;
> + low--;
> + }
> + *start = low;
> + }
> +
> + if (end) {
> + high = mid;
> + while (high < high_end) {
> + t = btf_type_by_id(btf, high+1);
> + name_buf = btf_name_by_offset(btf, t->name_off);
> + if (strcmp(name, name_buf))
> + break;
> + high++;
> + }
> + *end = high;
> }
>
this is an overcomplicated implementation, you need something like
find_linfo() implementation in kernel/bpf/log.c. Note how much shorter
and leaner it is.
I also don't think you need to return `end`. Given you always start
from start and linearly scan forward, you just need to make sure that
you never go beyond the BTF type array, for which you can use
btf_type_cnt(). So no need for doing this linear scan twice.
> + return mid;
> +}
> +
> +s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
> +{
> + const struct btf_type *t;
> + const char *tname;
> + int start, end;
> + s32 id, total;
> +
> + do {
> + if (btf->nr_types_sorted) {
> + /* binary search */
> + id = btf_find_by_name_bsearch(btf, name, &start, &end);
> + if (id > 0) {
> + while (start <= end) {
> + t = btf_type_by_id(btf, start);
> + if (BTF_INFO_KIND(t->info) == kind)
> + return start;
> + start++;
> + }
> + }
> + } else {
> + /* linear search */
> + total = btf_type_cnt(btf);
> + for (id = btf->base_btf ? btf->start_id : 1;
> + 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;
> + }
> + }
> + btf = btf->base_btf;
> + } while (btf);
> +
> return -ENOENT;
> }
>
> @@ -6141,6 +6221,53 @@ int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_ty
> return kctx_type_id;
> }
>
> +static int btf_check_sort(struct btf *btf, int start_id)
> +{
> + int i, n, nr_names = 0;
> +
> + n = btf_nr_types(btf);
> + for (i = start_id; i < n; i++) {
> + const struct btf_type *t;
> + const char *name;
> +
> + t = btf_type_by_id(btf, i);
> + if (!t)
> + return -EINVAL;
> +
> + name = btf_str_by_offset(btf, t->name_off);
> + if (!str_is_empty(name))
> + nr_names++;
> + }
> +
this loop makes zero sense to me, what are you trying to achieve with
it and why?
> + for (i = 0; i < nr_names - 1; i++) {
just start from start_id + 1, all the way to n, and check that sorting
invariant holds for all items
> + const struct btf_type *t1, *t2;
> + const char *s1, *s2;
> +
> + t1 = btf_type_by_id(btf, start_id + i);
> + if (!t1)
> + return -EINVAL;
> +
> + s1 = btf_str_by_offset(btf, t1->name_off);
> + if (str_is_empty(s1))
> + goto out;
> +
> + t2 = btf_type_by_id(btf, start_id + i + 1);
> + if (!t2)
> + return -EINVAL;
> +
> + s2 = btf_str_by_offset(btf, t2->name_off);
> + if (str_is_empty(s2))
> + goto out;
> +
> + if (strcmp(s1, s2) > 0)
> + goto out;
> + }
> +
> + btf->nr_types_sorted = nr_names;
> +out:
> + return 0;
> +}
[...]
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