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Date:   Tue, 17 Mar 2020 20:13:45 +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 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. 

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?

Thanks,
Song

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