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Date:   Tue, 17 Mar 2020 21:03:06 +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 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?

Thanks,
Daniel

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