Re: [PATCH bpf-next v2 1/1] libbpf: perfbuf: allow raw access to buffers

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On Sat, Jul 9, 2022 at 10:43 PM Jon Doron <arilou@xxxxxxxxx> wrote:
>
> I was referring to the following:
> https://github.com/libbpf/libbpf-rs/blob/master/libbpf-rs/src/perf_buffer.rs

How does your patch help libbpf-rs?

Please don't top post.

> Thanks,
> -- Jon.
>
> On Sun, Jul 10, 2022, 08:23 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>>
>> On Fri, Jul 8, 2022 at 7:54 PM Jon Doron <arilou@xxxxxxxxx> wrote:
>> >
>> > On 08/07/2022, Andrii Nakryiko wrote:
>> > >On Thu, Jul 7, 2022 at 11:04 PM Jon Doron <arilou@xxxxxxxxx> wrote:
>> > >>
>> > >> From: Jon Doron <jond@xxxxxx>
>> > >>
>> > >> Add support for writing a custom event reader, by exposing the ring
>> > >> buffer state, and allowing to set it's tail.
>> > >>
>> > >> Few simple examples where this type of needed:
>> > >> 1. perf_event_read_simple is allocating using malloc, perhaps you want
>> > >>    to handle the wrap-around in some other way.
>> > >> 2. Since perf buf is per-cpu then the order of the events is not
>> > >>    guarnteed, for example:
>> > >>    Given 3 events where each event has a timestamp t0 < t1 < t2,
>> > >>    and the events are spread on more than 1 CPU, then we can end
>> > >>    up with the following state in the ring buf:
>> > >>    CPU[0] => [t0, t2]
>> > >>    CPU[1] => [t1]
>> > >>    When you consume the events from CPU[0], you could know there is
>> > >>    a t1 missing, (assuming there are no drops, and your event data
>> > >>    contains a sequential index).
>> > >>    So now one can simply do the following, for CPU[0], you can store
>> > >>    the address of t0 and t2 in an array (without moving the tail, so
>> > >>    there data is not perished) then move on the CPU[1] and set the
>> > >>    address of t1 in the same array.
>> > >>    So you end up with something like:
>> > >>    void **arr[] = [&t0, &t1, &t2], now you can consume it orderely
>> > >>    and move the tails as you process in order.
>> > >> 3. Assuming there are multiple CPUs and we want to start draining the
>> > >>    messages from them, then we can "pick" with which one to start with
>> > >>    according to the remaining free space in the ring buffer.
>> > >>
>> > >
>> > >All the above use cases are sufficiently advanced that you as such an
>> > >advanced user should be able to write your own perfbuf consumer code.
>> > >There isn't a lot of code to set everything up, but then you get full
>> > >control over all the details.
>> > >
>> > >I don't see this API as a generally useful, it feels way too low-level
>> > >and special for inclusion in libbpf.
>> > >
>> >
>> > Hi Andrii,
>> >
>> > I understand, but I was still hoping you will be willing to expose this
>> > API.
>> > libbpf has very simple and nice binding to Rust and other languages,
>> > implementing one of those use cases in the bindings can make things much
>> > simpler than using some libc or syscall APIs, instead of enjoying all
>> > the simplicity that you get for free in libbpf.
>> >
>> > Hope you will be willing to reconsider :)
>>
>> The discussion would have been different if you mentioned that
>> motivation in the commit logs.
>> Please provide links to "Rust and other languages" code that
>> uses this api.



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