Re: [External] Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu_elem for percpu map

[Feng Zhou <zhoufeng.zf@bytedance.com>]

在 2022/5/10 上午11:15, Alexei Starovoitov 写道:
> On Mon, May 9, 2022 at 7:41 PM Feng Zhou <zhoufeng.zf@bytedance.com> wrote:
>> 在 2022/5/10 上午9:04, Yosry Ahmed 写道:
>>> On Mon, May 9, 2022 at 5:34 PM Andrii Nakryiko
>>> <andrii.nakryiko@gmail.com> wrote:
>>>> On Fri, May 6, 2022 at 7:49 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>>>> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>>>>>
>>>>> Trace some functions, such as enqueue_task_fair, need to access the
>>>>> corresponding cpu, not the current cpu, and bpf_map_lookup_elem percpu map
>>>>> cannot do it. So add bpf_map_lookup_percpu_elem to accomplish it for
>>>>> percpu_array_map, percpu_hash_map, lru_percpu_hash_map.
>>>>>
>>>>> The implementation method is relatively simple, refer to the implementation
>>>>> method of map_lookup_elem of percpu map, increase the parameters of cpu, and
>>>>> obtain it according to the specified cpu.
>>>>>
>>>> I don't think it's safe in general to access per-cpu data from another
>>>> CPU. I'd suggest just having either a ARRAY_OF_MAPS or adding CPU ID
>>>> as part of the key, if you need such a custom access pattern.
>>> I actually just sent an RFC patch series containing a similar patch
>>> for the exact same purpose. There are instances in the kernel where
>>> per-cpu data is accessed from other cpus (e.g.
>>> mem_cgroup_css_rstat_flush()). I believe, like any other variable,
>>> percpu data can be safe or not safe to access, based on the access
>>> pattern. It is up to the user to coordinate accesses to the variable.
>>>
>>> For example, in my use case, one of the accessors only reads percpu
>>> values of different cpus, so it should be safe. If a user accesses
>>> percpu data of another cpu without guaranteeing safety, they corrupt
>>> their own data. I understand that the main purpose of percpu data is
>>> lockless (and therefore fast) access, but in some use cases the user
>>> may be able to safely (and locklessly) access the data concurrently.
>>>
>> Regarding data security, I think users need to consider before using it,
>> such
>> as hook enqueue_task_fair, the function itself takes the rq lock of the
>> corresponding cpu, there is no problem, and the kernel only provides a
>> method,
>> like bpf_per_cpu_ptr and bpf_this_cpu_ptr, data security needs to be
>> guaranteed
>> by users in different scenarios, such as using bpf_spin_lock.
> Right. The new helper looks useful and is safe.
> Please add a selftest and respin.


Ok, will do. Thanks.