Re: [PATCH bpf-next v3 4/7] bpf: Introduce css open-coded iterator kfuncs

Chuyi Zhou
2023-09-28T02:52:13Z

Hello,

在 2023/9/28 07:24, Andrii Nakryiko 写道:
> On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> This Patch adds kfuncs bpf_iter_css_{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}.
>> css_iter can be used to:
>>
>> 1) iterating a sepcific cgroup tree with pre/post/up order
>>
>> 2) iterating cgroup_subsystem in BPF Prog, like
>> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
>>
>> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
>> parameters defining iteration order and starting css. Here we also reuse
>> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
>> BPF_CGROUP_ITER_ANCESTORS_UP enums.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>> kernel/bpf/cgroup_iter.c | 57 +++++++++++++++++++
>> kernel/bpf/helpers.c | 3 +
>> .../testing/selftests/bpf/bpf_experimental.h | 6 ++
>> 3 files changed, 66 insertions(+)
>>
>> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
>> index 810378f04fbc..ebc3d9471f52 100644
>> --- a/kernel/bpf/cgroup_iter.c
>> +++ b/kernel/bpf/cgroup_iter.c
>> @@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
>> }
>>
>> late_initcall(bpf_cgroup_iter_init);
>> +
>> +struct bpf_iter_css {
>> + __u64 __opaque[2];
>> + __u32 __opaque_int[1];
>> +} __attribute__((aligned(8)));
>> +
>
> same as before, __opaque[3] only
>
>
>> +struct bpf_iter_css_kern {
>> + struct cgroup_subsys_state *start;
>> + struct cgroup_subsys_state *pos;
>> + int order;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
>> + struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)
>
> Similarly, I wonder if we should go for a more generic "flags" argument?
>
>> +{
>> + struct bpf_iter_css_kern *kit = (void *)it;
>
> empty line
>
>> + kit->start = NULL;
>> + BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
>> + BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
>
> please move this up before kit->start assignment, and separate by empty lines
>
>> + switch (order) {
>> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
>> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
>> + case BPF_CGROUP_ITER_ANCESTORS_UP:
>> + break;
>> + default:
>> + return -EINVAL;
>> + }
>> +
>> + kit->start = start;
>> + kit->pos = NULL;
>> + kit->order = order;
>> + return 0;
>> +}
>> +
>> +__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
>> +{
>> + struct bpf_iter_css_kern *kit = (void *)it;
>
> empty line
>
>> + if (!kit->start)
>> + return NULL;
>> +
>> + switch (kit->order) {
>> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
>> + kit->pos = css_next_descendant_pre(kit->pos, kit->start);
>> + break;
>> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
>> + kit->pos = css_next_descendant_post(kit->pos, kit->start);
>> + break;
>> + default:
>
> we know it's BPF_CGROUP_ITER_ANCESTORS_UP, so why not have that here explicitly?
>
>> + kit->pos = kit->pos ? kit->pos->parent : kit->start;
>> + }
>> +
>> + return kit->pos;
>
> wouldn't this implementation never return the "start" css? is that intentional?
>

Thanks for the review.

This implementation actually would return the "start" css.

1. BPF_CGROUP_ITER_DESCENDANTS_PRE:
1.1 when we first call next(), css_next_descendant_pre(NULL, kit->start)
will return kit->start.
1.2 second call next(), css_next_descendant_pre(kit->start, kit->start)
would return a first valid child under kit->start with pre-order
1.3 third call next, css_next_descendant_pre(last_valid_child,
kit->start) would return the next valid child
...
util css_next_descendant_pre return a NULL pointer, which means we have
visited all valid child including "start" css itself.

The above logic is equal to macro 'css_for_each_descendant_pre' in kernel.

Same, BPF_CGROUP_ITER_DESCENDANTS_POST is equal to macro
'css_for_each_descendant_post' which would return 'start' css when we
have visited all valid child.

2. BPF_CGROUP_ITER_ANCESTORS_UP
2.1 when we fisrt call next(), kit->pos is NULL, and we would return
kit->start.

The selftest in patch7 whould check:
1. when we use BPF_CGROUP_ITER_DESCENDANTS_PRE to iterate a cgroup tree,
the first cgroup we visted should be root('start') cgroup.
2. when we use BPF_CGROUP_ITER_DESCENDANTS_POST to iterate a cgroup
tree, the last cgroup we visited should be root('start') cgroup.

Am I miss something important?

Thanks.