netdev - Re: [PATCH v2 bpf-next] bpf: sharing bpf runtime stats with /dev/bpf

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Date:   Wed, 18 Mar 2020 06:33:26 +0000
From:   Song Liu <songliubraving@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "yzaikin@...gle.com" <yzaikin@...gle.com>
Subject: Re: [PATCH v2 bpf-next] bpf: sharing bpf runtime stats with
 /dev/bpf_stats



> On Mar 17, 2020, at 4:08 PM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Mar 17, 2020, at 2:47 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>>>> 
>>>> Hm, true as well. Wouldn't long-term extending "bpftool prog profile" fentry/fexit
>>>> programs supersede this old bpf_stats infrastructure? Iow, can't we implement the
>>>> same (or even more elaborate stats aggregation) in BPF via fentry/fexit and then
>>>> potentially deprecate bpf_stats counters?
>>> I think run_time_ns has its own value as a simple monitoring framework. We can
>>> use it in tools like top (and variations). It will be easier for these tools to
>>> adopt run_time_ns than using fentry/fexit.
>> 
>> Agree that this is easier; I presume there is no such official integration today
>> in tools like top, right, or is there anything planned?
> 
> Yes, we do want more supports in different tools to increase the visibility. 
> Here is the effort for atop: https://github.com/Atoptool/atop/pull/88 .
> 
> I wasn't pushing push hard on this one mostly because the sysctl interface requires 
> a user space "owner". 
> 
>> 
>>> On the other hand, in long term, we may include a few fentry/fexit based programs
>>> in the kernel binary (or the rpm), so that these tools can use them easily. At
>>> that time, we can fully deprecate run_time_ns. Maybe this is not too far away?
>> 
>> Did you check how feasible it is to have something like `bpftool prog profile top`
>> which then enables fentry/fexit for /all/ existing BPF programs in the system? It
>> could then sort the sample interval by run_cnt, cycles, cache misses, aggregated
>> runtime, etc in a top-like output. Wdyt?
> 
> I wonder whether we can achieve this with one bpf prog (or a trampoline) that covers
> all BPF programs, like a trampoline inside __BPF_PROG_RUN()? 
> 
> For long term direction, I think we could compare two different approaches: add new 
> tools (like bpftool prog profile top) vs. add BPF support to existing tools. The 
> first approach is easier. The latter approach would show BPF information to users
> who are not expecting BPF programs in the systems. For many sysadmins, seeing BPF
> programs in top/ps, and controlling them via kill is more natural than learning
> bpftool. What's your thought on this? 

More thoughts on this. 

If we have a special trampoline that attach to all BPF programs at once, we really 
don't need the run_time_ns stats anymore. Eventually, tools that monitor BPF 
programs will depend on libbpf, so using fentry/fexit to monitor BPF programs doesn't
introduce extra dependency. I guess we also need a way to include BPF program in 
libbpf. 

To summarize this plan, we need:

1) A global trampoline that attaches to all BPF programs at once;
2) Embed fentry/fexit program in libbpf, which will be used by tools for monitoring;
3) BPF helpers to read time, which replaces current run_time_ns. 

Does this look reasonable?

Thanks,
Song