Messages in this thread | | | Date | Tue, 10 May 2022 13:52:52 +0800 | Subject | Re: [External] Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu_elem for percpu map | From | Feng Zhou <> |
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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.
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