linux-kernel - Re: [PATCH] sched_ext: Use BTF_ID to resolve task

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Message-ID: <CAADnVQJT-wWpMJH-CiNEKuUHLwO0dKvjOaUJbzw5GGG0EqgRAA@mail.gmail.com>
Date: Wed, 16 Oct 2024 19:12:23 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tianchen Ding <dtcccc@...ux.alibaba.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>, 
	David Vernet <void@...ifault.com>, Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] sched_ext: Use BTF_ID to resolve task_struct

On Wed, Oct 16, 2024 at 6:57 PM Tianchen Ding <dtcccc@...ux.alibaba.com> wrote:
>
> On 2024/10/17 00:57, Alexei Starovoitov wrote:
> > On Tue, Oct 15, 2024 at 7:42 PM Tianchen Ding <dtcccc@...ux.alibaba.com> wrote:
> >>
> >> Save the searching time during bpf_scx_init.
> >>
> >> Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
> >> ---
> >>   kernel/sched/ext.c | 12 +++---------
> >>   1 file changed, 3 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> >> index 609b9fb00d6f..1d11a96eefb8 100644
> >> --- a/kernel/sched/ext.c
> >> +++ b/kernel/sched/ext.c
> >> @@ -5343,7 +5343,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
> >>
> >>   extern struct btf *btf_vmlinux;
> >>   static const struct btf_type *task_struct_type;
> >> -static u32 task_struct_type_id;
> >> +BTF_ID_LIST_SINGLE(task_struct_btf_ids, struct, task_struct);
> >>
> >>   static bool set_arg_maybe_null(const char *op, int arg_n, int off, int size,
> >>                                 enum bpf_access_type type,
> >> @@ -5395,7 +5395,7 @@ static bool set_arg_maybe_null(const char *op, int arg_n, int off, int size,
> >>                   */
> >>                  info->reg_type = PTR_MAYBE_NULL | PTR_TO_BTF_ID | PTR_TRUSTED;
> >>                  info->btf = btf_vmlinux;
> >> -               info->btf_id = task_struct_type_id;
> >> +               info->btf_id = task_struct_btf_ids[0];
> >>
> >>                  return true;
> >>          }
> >> @@ -5547,13 +5547,7 @@ static void bpf_scx_unreg(void *kdata, struct bpf_link *link)
> >>
> >>   static int bpf_scx_init(struct btf *btf)
> >>   {
> >> -       s32 type_id;
> >> -
> >> -       type_id = btf_find_by_name_kind(btf, "task_struct", BTF_KIND_STRUCT);
> >> -       if (type_id < 0)
> >> -               return -EINVAL;
> >> -       task_struct_type = btf_type_by_id(btf, type_id);
> >> -       task_struct_type_id = type_id;
> >> +       task_struct_type = btf_type_by_id(btf, task_struct_btf_ids[0]);
> >
> > Good optimization, but it's also unnecessary.
> >
> > btf_id is already in btf_tracing_ids[BTF_TRACING_TYPE_TASK].
>
> Get it. Thanks!
>
> BTW, do you think we should add a zero check for
> btf_tracing_ids[BTF_TRACING_TYPE_TASK] here?
> task_struct should always be valid. If something wrong, resolve_btfids will also
> throw a warning. I'm not sure whether to add a sanity check here.

Definitely shouldn't add run-time checks.
Build check may work, but feels overkill at this point.