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

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Date:   Wed, 7 Nov 2018 00:26:16 +0000
From:   Alexei Starovoitov <ast@...com>
To:     David Ahern <dsahern@...il.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 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.