netdev - Re: [RFC Patch bpf-next] bpf: introduce bpf timer

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Message-ID: <E0D5B076-A726-4845-8F12-640BAA853525@fb.com>
Date:   Thu, 1 Apr 2021 20:17:47 +0000
From:   Song Liu <songliubraving@...com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <bpf@...r.kernel.org>,
        "duanxiongchun@...edance.com" <duanxiongchun@...edance.com>,
        "wangdongdong.6@...edance.com" <wangdongdong.6@...edance.com>,
        Muchun Song <songmuchun@...edance.com>,
        "Cong Wang" <cong.wang@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "Martin Lau" <kafai@...com>, Yonghong Song <yhs@...com>
Subject: Re: [RFC Patch bpf-next] bpf: introduce bpf timer



> On Apr 1, 2021, at 10:28 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>> 
>>> From: Cong Wang <cong.wang@...edance.com>
>>> 
>>> (This patch is still in early stage and obviously incomplete. I am sending
>>> it out to get some high-level feedbacks. Please kindly ignore any coding
>>> details for now and focus on the design.)
>> 
>> Could you please explain the use case of the timer? Is it the same as
>> earlier proposal of BPF_MAP_TYPE_TIMEOUT_HASH?
>> 
>> Assuming that is the case, I guess the use case is to assign an expire
>> time for each element in a hash map; and periodically remove expired
>> element from the map.
>> 
>> If this is still correct, my next question is: how does this compare
>> against a user space timer? Will the user space timer be too slow?
> 
> Yes, as I explained in timeout hashmap patchset, doing it in user-space
> would require a lot of syscalls (without batching) or copying (with batching).
> I will add the explanation here, in case people miss why we need a timer.

How about we use a user space timer to trigger a BPF program (e.g. use 
BPF_PROG_TEST_RUN on a raw_tp program); then, in the BPF program, we can 
use bpf_for_each_map_elem and bpf_map_delete_elem to scan and update the 
map? With this approach, we only need one syscall per period. 

Thanks,
Song