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

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Date:   Tue, 17 Mar 2020 22:47:00 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <songliubraving@...com>
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 3/17/20 9:13 PM, Song Liu wrote:
>> On Mar 17, 2020, at 1:03 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 3/17/20 8:54 PM, Song Liu wrote:
>>>> On Mar 17, 2020, at 12:30 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>>>> On 3/16/20 9:33 PM, Song Liu wrote:
>>>>> Currently, sysctl kernel.bpf_stats_enabled controls BPF runtime stats.
>>>>> Typical userspace tools use kernel.bpf_stats_enabled as follows:
>>>>>    1. Enable kernel.bpf_stats_enabled;
>>>>>    2. Check program run_time_ns;
>>>>>    3. Sleep for the monitoring period;
>>>>>    4. Check program run_time_ns again, calculate the difference;
>>>>>    5. Disable kernel.bpf_stats_enabled.
>>>>> The problem with this approach is that only one userspace tool can toggle
>>>>> this sysctl. If multiple tools toggle the sysctl at the same time, the
>>>>> measurement may be inaccurate.
>>>>> To fix this problem while keep backward compatibility, introduce a new
>>>>> bpf command BPF_ENABLE_RUNTIME_STATS. On success, this command enables
>>>>> run_time_ns stats and returns a valid fd.
>>>>> With BPF_ENABLE_RUNTIME_STATS, user space tool would have the following
>>>>> flow:
>>>>>    1. Get a fd with BPF_ENABLE_RUNTIME_STATS, and make sure it is valid;
>>>>>    2. Check program run_time_ns;
>>>>>    3. Sleep for the monitoring period;
>>>>>    4. Check program run_time_ns again, calculate the difference;
>>>>>    5. Close the fd.
>>>>> Signed-off-by: Song Liu <songliubraving@...com>
>>>>
>>>> Hmm, I see no relation to /dev/bpf_stats anymore, yet the subject still talks
>>>> about it?
>>> My fault. Will fix..
>>>> Also, should this have bpftool integration now that we have `bpftool prog profile`
>>>> support? Would be nice to then fetch the related stats via bpf_prog_info, so users
>>>> can consume this in an easy way.
>>> We can add "run_time_ns" as a metric to "bpftool prog profile". But the
>>> mechanism is not the same though. Let me think about this.
>>
>> 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?

> 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?

Thanks,
Daniel