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

[Andrii Nakryiko]

On Thu, Dec 5, 2019 at 4:13 PM Daniel Xu <dxu@xxxxxxxxx> 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.
>
> Some notes:
> * We use `__u64 entries[BPF_MAX_LBR_ENTRIES * 3]` instead of
>   `struct perf_branch_entry[BPF_MAX_LBR_ENTRIES]` because checkpatch.pl
>   warns about including a uapi header from another uapi header
>
> * We define BPF_MAX_LBR_ENTRIES as 32 (instead of using the value from
>   arch/x86/events/perf_events.h) because including arch specific headers
>   seems wrong and could introduce circular header includes.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  include/uapi/linux/bpf_perf_event.h |  5 ++++
>  kernel/trace/bpf_trace.c            | 39 +++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/include/uapi/linux/bpf_perf_event.h b/include/uapi/linux/bpf_perf_event.h
> index eb1b9d21250c..dc87e3d50390 100644
> --- a/include/uapi/linux/bpf_perf_event.h
> +++ b/include/uapi/linux/bpf_perf_event.h
> @@ -10,10 +10,15 @@
>
>  #include <asm/bpf_perf_event.h>
>
> +#define BPF_MAX_LBR_ENTRIES 32
> +
>  struct bpf_perf_event_data {
>         bpf_user_pt_regs_t regs;
>         __u64 sample_period;
>         __u64 addr;
> +       __u64 nr_lbr;
> +       /* Cast to struct perf_branch_entry* before using */
> +       __u64 entries[BPF_MAX_LBR_ENTRIES * 3];
>  };
>

I wonder if instead of hard-coding this in bpf_perf_event_data, could
we achieve this and perhaps even more flexibility by letting users
access underlying bpf_perf_event_data_kern and use CO-RE to read
whatever needs to be read from perf_sample_data, perf_event, etc?
Would that work?

>  #endif /* _UAPI__LINUX_BPF_PERF_EVENT_H__ */
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ffc91d4935ac..96ba7995b3d7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c

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