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

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Message-ID: <CAM_iQpVtxgZNeqh4_Pqftc3D163JnRvP3AZRuFrYNeyWLgVBVA@mail.gmail.com>
Date:   Mon, 26 Apr 2021 16:00:09 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...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>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [RFC Patch bpf-next] bpf: introduce bpf timer

Hi, Alexei

On Wed, Apr 14, 2021 at 9:25 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Apr 14, 2021 at 9:02 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > Then how do you prevent prog being unloaded when the timer callback
> > is still active?
>
> As I said earlier:
> "
> If prog refers such hmap as above during prog free the kernel does
> for_each_map_elem {if (elem->opaque) del_timer().}
> "

I have discussed this with my colleagues, sharing timers among different
eBPF programs is a must-have feature for conntrack.

For conntrack, we need to attach two eBPF programs, one on egress and
one on ingress. They share a conntrack table (an eBPF map), and no matter
we use a per-map or per-entry timer, updating the timer(s) could happen
on both sides, hence timers must be shared for both.

So, your proposal we discussed does not work well for this scenario. The
proposal in my RFC should still work. Please let me know if you have any
better ideas.

Thanks!