Re: [PATCH bpf-next v2 4/6] bpf: Introduce css_descendant open-coded iterator kfuncs

[Chuyi Zhou <zhouchuyi@bytedance.com>]

Hello.

在 2023/9/15 07:26, Andrii Nakryiko 写道:
> On Tue, Sep 12, 2023 at 12:02 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> This Patch adds kfuncs bpf_iter_css_{pre,post}_{new,next,destroy} which
>> allow creation and manipulation of struct bpf_iter_css in open-coded
>> iterator style. These kfuncs actually wrapps css_next_descendant_{pre,
>> post}. BPF programs can use these kfuncs through bpf_for_each macro for
>> iteration of all descendant css under a root css.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>>   include/uapi/linux/bpf.h       |  8 +++++
>>   kernel/bpf/helpers.c           |  6 ++++
>>   kernel/bpf/task_iter.c         | 53 ++++++++++++++++++++++++++++++++++
>>   tools/include/uapi/linux/bpf.h |  8 +++++
>>   tools/lib/bpf/bpf_helpers.h    | 12 ++++++++
>>   5 files changed, 87 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index befa55b52e29..57760afc13d0 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -7326,4 +7326,12 @@ struct bpf_iter_process {
>>          __u64 __opaque[1];
>>   } __attribute__((aligned(8)));
>>
>> +struct bpf_iter_css_pre {
>> +       __u64 __opaque[2];
>> +} __attribute__((aligned(8)));
>> +
>> +struct bpf_iter_css_post {
>> +       __u64 __opaque[2];
>> +} __attribute__((aligned(8)));
>> +
>>   #endif /* _UAPI__LINUX_BPF_H__ */
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 9b7d2c6f99d1..ca1f6404af9e 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2510,6 +2510,12 @@ BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
>>   BTF_ID_FLAGS(func, bpf_iter_process_new, KF_ITER_NEW)
>>   BTF_ID_FLAGS(func, bpf_iter_process_next, KF_ITER_NEXT | KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_process_destroy, KF_ITER_DESTROY)
>> +BTF_ID_FLAGS(func, bpf_iter_css_pre_new, KF_ITER_NEW)
>> +BTF_ID_FLAGS(func, bpf_iter_css_pre_next, KF_ITER_NEXT | KF_RET_NULL)
>> +BTF_ID_FLAGS(func, bpf_iter_css_pre_destroy, KF_ITER_DESTROY)
>> +BTF_ID_FLAGS(func, bpf_iter_css_post_new, KF_ITER_NEW)
>> +BTF_ID_FLAGS(func, bpf_iter_css_post_next, KF_ITER_NEXT | KF_RET_NULL)
>> +BTF_ID_FLAGS(func, bpf_iter_css_post_destroy, KF_ITER_DESTROY)
>>   BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>>   BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>>   BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 9d1927dc3a06..8963fc779b87 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -880,6 +880,59 @@ __bpf_kfunc void bpf_iter_process_destroy(struct bpf_iter_process *it)
>>   {
>>   }
>>
>> +struct bpf_iter_css_kern {
>> +       struct cgroup_subsys_state *root;
>> +       struct cgroup_subsys_state *pos;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_css_pre_new(struct bpf_iter_css_pre *it,
>> +               struct cgroup_subsys_state *root)
> 
> similar to my comment on previous patches, please see
> kernel/bpf/cgroup_iter.c for iter/cgroup iterator program. Let's stay
> consistent. We have one iterator that accepts parameters defining
> iteration order and starting cgroup. Unless there are some technical
> reasons we can't follow similar approach with this open-coded iter,
> let's use the same approach. We can even reuse
> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
> BPF_CGROUP_ITER_ANCESTORS_UP enums.
> 

I know your concern. It would be nice if we keep consistent with 
kernel/bpf/cgroup_iter.c

But this patch actually want to support iterating css 
(cgroup_subsys_state) not cgroup (css is more low lever).
With css_iter we can do something like 
"for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre"
in BPF Progs which is hard for cgroup_iter. In the future we can use 
this iterator to plug some customizable policy in other resource control 
system.

BTW, what I did in RFC actually very similar with the approach of 
cgroup_iter. 
(https://lore.kernel.org/all/20230827072057.1591929-4-zhouchuyi@bytedance.com/).

Thanks.