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

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Message-ID: <fa9600d8-2a6c-4c74-8e42-31d669c06b59@linux.alibaba.com>
Date: Thu, 17 Oct 2024 09:57:17 +0800
From: Tianchen Ding <dtcccc@...ux.alibaba.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.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 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.