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. > Signed-off-by: Jon Doron <jond@xxxxxx> > --- > tools/lib/bpf/libbpf.c | 40 ++++++++++++++++++++++++++++++++++++++++ > tools/lib/bpf/libbpf.h | 25 +++++++++++++++++++++++++ > tools/lib/bpf/libbpf.map | 2 ++ > 3 files changed, 67 insertions(+) > [...]
