Re: [PATCH bpf-next v3 0/1] libbpf: perfbuf expose ring buffer

[Yonghong Song <yhs@fb.com>]

On 7/15/22 7:18 AM, Jon Doron wrote:
> From: Jon Doron <jond@wiz.io>
> 
> Add support for writing a custom event reader, by exposing the ring
> buffer.
> 
> 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.
> 
> Jon Doron (1):
>    libbpf: perfbuf: Add API to get the ring buffer
> 
>   tools/lib/bpf/libbpf.c   | 26 ++++++++++++++++++++++++++
>   tools/lib/bpf/libbpf.h   |  2 ++
>   tools/lib/bpf/libbpf.map |  1 +
>   3 files changed, 29 insertions(+)
> 

This is a patch set with a single patch. There is no need for
the cover letter. Further the cover letter description is
identical to the commit message of the first patch.