linux-kernel - Re: [RFC perf,bpf 5/5] perf util: generate bpf_prog_info

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Message-ID: <7d0b100b-a64b-0d30-8ec0-2689ef44423d@gmail.com>
Date:   Tue, 6 Nov 2018 17:44:13 -0700
From:   David Ahern <dsahern@...il.com>
To:     Alexei Starovoitov <ast@...com>, David Miller <davem@...emloft.net>
Cc:     Song Liu <songliubraving@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "acme@...nel.org" <acme@...nel.org>
Subject: Re: [RFC perf,bpf 5/5] perf util: generate bpf_prog_info_event for
 short living bpf programs

On 11/6/18 5:26 PM, Alexei Starovoitov wrote:
> On 11/6/18 4:23 PM, David Ahern wrote:
>> On 11/6/18 5:13 PM, Alexei Starovoitov wrote:
>>> On 11/6/18 3:36 PM, David Miller wrote:
>>>> From: Alexei Starovoitov <ast@...com>
>>>> Date: Tue, 6 Nov 2018 23:29:07 +0000
>>>>
>>>>> I think concerns with perf overhead from collecting bpf events
>>>>> are unfounded.
>>>>> I would prefer for this flag to be on by default.
>>>>
>>>> I will sit in userspace looping over bpf load/unload and see how the
>>>> person trying to monitor something else with perf feels about that.
>>>>
>>>> Really, it is inappropriate to turn this on by default, I completely
>>>> agree with David Ahern.
>>>>
>>>> It's hard enough, _AS IS_, for me to fight back all of the bloat that
>>>> is in perf right now and get it back to being able to handle simple
>>>> full workloads without dropping events..
>>>
>>> It's a separate perf thread and separate event with its own epoll.
>>> I don't see how it can affect main event collection.
>>> Let's put it this way. If it does affect somehow, then yes,
>>> it should not be on. If it is not, there is no downside to keep it on.
>>> Typical user expects to type 'perf record' and see everything that
>>> is happening on the system. Right now short lived bpf programs
>>> will not be seen. How user suppose to even know when to use the flag?
>>
>> The default is profiling where perf record collects task events and
>> periodic samples. So for the default record/report, the bpf load /
>> unload events are not relevant.
> 
> Exactly the opposite.
> It's for default 'perf record' collection of periodic samples.
> It can be off for -e collection. That's easy.
> 

So one use case is profiling bpf programs. I was also considering the
auditing discussion from some weeks ago which I thought the events are
also targeting.

As for the overhead, I did not see a separate thread getting spun off
for the bpf events, so the events are processed inline for this RFC set.