netdev - Re: [PATCH bpf-next 2/3] bpf: emit RECORD_MMAP events for bpf prog load/unload

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Date:   Wed, 17 Oct 2018 08:09:40 -0700
From:   David Ahern <dsahern@...il.com>
To:     Song Liu <liu.song.a23@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>, acme@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, kernel-team@...com
Subject: Re: [PATCH bpf-next 2/3] bpf: emit RECORD_MMAP events for bpf prog
 load/unload

On 10/16/18 11:43 PM, Song Liu wrote:
> I agree that processing events while recording has significant overhead.
> In this case, perf user space need to know details about the the jited BPF
> program. It is impossible to pass all these details to user space through
> the relatively stable ring_buffer API. Therefore, some processing of the
> data is necessary (get bpf prog_id from ring buffer, and then fetch program
> details via BPF_OBJ_GET_INFO_BY_FD.
> 
> I have some idea on processing important data with relatively low overhead.
> Let me try implement it.
> 

As I understand it, you want this series:

 kernel: add event to perf buffer on bpf prog load

 userspace: perf reads the event and grabs information about the program
from the fd

Is that correct?

Userpsace is not awakened immediately when an event is added the the
ring. It is awakened once the number of events crosses a watermark. That
means there is an unknown - and potentially long - time window where the
program can be unloaded before perf reads the event.

So no matter what you do expecting perf record to be able to process the
event quickly is an unreasonable expectation.