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

From: Cong Wang <xiyou.wangcong@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: "open list:BPF (Safe dynamic programs and tools)" 
	<netdev@vger.kernel.org>,
	"open list:BPF (Safe dynamic programs and tools)" 
	<bpf@vger.kernel.org>,
	"duanxiongchun@bytedance.com" <duanxiongchun@bytedance.com>,
	"wangdongdong.6@bytedance.com" <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 Lau <kafai@fb.com>,
	Yonghong Song <yhs@fb.com>
Subject: Re: [RFC Patch bpf-next] bpf: introduce bpf timer
Date: Thu, 1 Apr 2021 10:28:12 -0700	[thread overview]
Message-ID: <CAM_iQpW-cuiYsPsu4mYZxZ1Oixffu2pV1TFg1c+eg9XT3wWwPQ@mail.gmail.com> (raw)
In-Reply-To: <B42B247A-4D0B-4DE9-B4D3-0C452472532D@fb.com>

On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >
> > From: Cong Wang <cong.wang@bytedance.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.

>
> >
> > This patch introduces a bpf timer map and a syscall to create bpf timer
> > from user-space.
> >
> > The reason why we have to use a map is because the lifetime of a timer,
> > without a map, we have to delete the timer before exiting the eBPF program,
> > this would significately limit its use cases. With a map, the timer can
> > stay as long as the map itself and can be actually updated via map update
> > API's too, where the key is the timer ID and the value is the timer expire
> > timer.
> >
> > Timer creation is not easy either. In order to prevent users creating a
> > timer but not adding it to a map, we have to enforce this in the API which
> > takes a map parameter and adds the new timer into the map in one shot.
>
> I think we don't have to address "creating a timer but not adding it to a map"
> problem in the kernel. If the user forgot it, the user should debug it.

Good point. Initially the timer is created in kernel-space, now it is in user
space, so it is probably fine to create it without a map. But we would have
to provide more syscalls for users to manage the timer, so using a map
still has an advantage of not adding more syscalls.

Thanks.