Re: [PATCH bpf] bpf: Add LBR data to BPF_PROG_TYPE_PERF_EVENT prog context

[Martin Lau]

On Thu, Dec 05, 2019 at 04:12:26PM -0800, Daniel Xu wrote:
> Last-branch-record is an intel CPU feature that can be configured to
> record certain branches that are taken during code execution. This data
> is particularly interesting for profile guided optimizations. perf has
> had LBR support for a while but the data collection can be a bit coarse
> grained.
> 
> We (Facebook) have recently run a lot of experiments with feeding
> filtered LBR data to various PGO pipelines. We've seen really good
> results (+2.5% throughput with lower cpu util and lower latency) by
> feeding high request latency LBR branches to the compiler on a
> request-oriented service. We used bpf to read a special request context
> ID (which is how we associate branches with latency) from a fixed
> userspace address. Reading from the fixed address is why bpf support is
> useful.
> 
> Aside from this particular use case, having LBR data available to bpf
> progs can be useful to get stack traces out of userspace applications
> that omit frame pointers.
> 
> This patch adds support for LBR data to bpf perf progs.
Please re-submit when bpf-next opens.