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

From: Cong Wang <xiyou.wangcong@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Linux Kernel Network Developers <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>,
	Xiongchun Duan <duanxiongchun@bytedance.com>,
	Dongdong Wang <wangdongdong.6@bytedance.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Cong Wang <cong.wang@bytedance.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	Pedro Tammela <pctammela@mojatatu.com>,
	Jamal Hadi Salim <jhs@mojatatu.com>
Subject: Re: [RFC Patch bpf-next] bpf: introduce bpf timer
Date: Mon, 26 Apr 2021 16:37:19 -0700	[thread overview]
Message-ID: <CAM_iQpWDhoY_msU=AowHFq3N3OuQpvxd2ADP_Z+gxBfGduhrPA@mail.gmail.com> (raw)
In-Reply-To: <CAADnVQLFehCeQRbwEQ9VM-=Y3V3es2Ze8gFPs6cZHwNH0Ct7vw@mail.gmail.com>

On Mon, Apr 26, 2021 at 4:05 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon, Apr 26, 2021 at 4:00 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >
> > Hi, Alexei
> >
> > On Wed, Apr 14, 2021 at 9:25 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Wed, Apr 14, 2021 at 9:02 PM Cong Wang <xiyou.wangcong@gmail.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.
>
> why? The timer inside the map element will be shared just fine.
> Just like different progs can see the same map value.

Hmm? In the above quotes from you, you suggested removing all the
timers installed by one eBPF program when it is freed, but they could be
still running independent of which program installs them.

In other words, timers are independent of other eBPF programs, so
they should not have an owner. With your proposal, the owner of a timer
is the program which contains the subprog (or callback) of the timer.
With my proposal, the timer callback is a standalone program hence has
no owner.

>
> Also if your colleagues have something to share they should be
> posting to the mailing list. Right now you're acting as a broken phone
> passing info back and forth and the knowledge gets lost.
> Please ask your colleagues to participate online.

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

Thanks.