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Add support for writing a custom event reader, by exposing the ring buffer. With the new API perf_buffer__buffer() you will get access to the raw mmaped()'ed per-cpu underlying memory of the ring buffer. This region contains both the perf buffer data and header (struct perf_event_mmap_page), which manages the ring buffer state (head/tail positions, when accessing the head/tail position it's important to take into consideration SMP). With this type of low level access one can implement different types of consumers here are few simple examples where this API helps with: 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. Signed-off-by: Jon Doron <jond@wiz.io> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Link: https://lore.kernel.org/bpf/20220715181122.149224-1-arilou@gmail.com |
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