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

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Message-ID: <34af20ce-b749-7e37-2658-9aca6304614a@mojatatu.com>
Date:   Tue, 27 Apr 2021 07:52:02 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Dongdong Wang <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 KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [RFC Patch bpf-next] bpf: introduce bpf timer

On 2021-04-26 10:01 p.m., Alexei Starovoitov wrote:

[..]
>>
>> They are already in CC from the very beginning. And our use case is
>> public, it is Cilium conntrack:
>> https://github.com/cilium/cilium/blob/master/bpf/lib/conntrack.h
>>
>> The entries of the code are:
>> https://github.com/cilium/cilium/blob/master/bpf/bpf_lxc.c
>>
>> The maps for conntrack are:
>> https://github.com/cilium/cilium/blob/master/bpf/lib/conntrack_map.h
> 
> If that's the only goal then kernel timers are not needed.
> cilium conntrack works well as-is.

IIRC, the original patch from Cong was driven by need to scale said
conntracking in presence of large number of flows.
The arguement i heard from Cong is LRU doesnt scale in such a setup.

I would argue timers generally are useful for a variety of house
keeping purposes and they are currently missing from ebpf. This
despite Cong's use case.
Currently things in the datapath are triggered by either packets
showing up or from a control plane perspective by user space polling.

Our use case (honestly, not that it matters to justify why we need
timers) is we want to periodically, if some condition is met in the
kernel, to send unsolicited housekeeping events to user space.

Hope that helps.

cheers,
jamal