On 1/25/20 2:31 PM, Daniel Xu wrote:
> Branch records are a 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
> branch record support for a while but the data collection can be a bit
> coarse grained.
>
> We (Facebook) have seen in experiments that associating metadata with
> branch records can improve results (after postprocessing). We generally
> use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
> support for branch records is useful.
>
> Aside from this particular use case, having branch data available to bpf
> progs can be useful to get stack traces out of userspace applications
> that omit frame pointers.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
> include/uapi/linux/bpf.h | 25 +++++++++++++++++++++++-
> kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 65 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f1d74a2bd234..332aa433d045 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2892,6 +2892,25 @@ union bpf_attr {
> * Obtain the 64bit jiffies
> * Return
> * The 64 bit jiffies
> + *
> + * int bpf_read_branch_records(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size, u64 flags)
> + * Description
> + * For an eBPF program attached to a perf event, retrieve the
> + * branch records (struct perf_branch_entry) associated to *ctx*
> + * and store it in the buffer pointed by *buf* up to size
> + * *buf_size* bytes.
> + *
> + * The *flags* can be set to **BPF_F_GET_BRANCH_RECORDS_SIZE** to
> + * instead return the number of bytes required to store all the
> + * branch entries. If this flag is set, *buf* may be NULL.
> + * Return
> + * On success, number of bytes written to *buf*. On error, a
> + * negative value.
> + *
> + * **-EINVAL** if arguments invalid or **buf_size** not a multiple
> + * of sizeof(struct perf_branch_entry).
> + *
> + * **-ENOENT** if architecture does not support branch records.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3012,7 +3031,8 @@ union bpf_attr {
> FN(probe_read_kernel_str), \
> FN(tcp_send_ack), \
> FN(send_signal_thread), \
> - FN(jiffies64),
> + FN(jiffies64), \
> + FN(read_branch_records),
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> * function eBPF program intends to call
> @@ -3091,6 +3111,9 @@ enum bpf_func_id {
> /* BPF_FUNC_sk_storage_get flags */
> #define BPF_SK_STORAGE_GET_F_CREATE (1ULL << 0)
>
> +/* BPF_FUNC_read_branch_records flags. */
> +#define BPF_F_GET_BRANCH_RECORDS_SIZE (1ULL << 0)
> +
> /* Mode for BPF_FUNC_skb_adjust_room helper. */
> enum bpf_adj_room_mode {
> BPF_ADJ_ROOM_NET,
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 19e793aa441a..5a0ab7c9a1dc 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1028,6 +1028,45 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
> .arg3_type = ARG_CONST_SIZE,
> };
>
> +BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
> + void *, buf, u32, size, u64, flags)
> +{
> + struct perf_branch_stack *br_stack = ctx->data->br_stack;
> + u32 br_entry_size = sizeof(struct perf_branch_entry);
> + u32 to_copy;
> +
> +#ifndef CONFIG_X86
> + return -ENOENT;
> +#endif
For non x86 platform, we will get a lot of compiler warning for
unused variables?
> +
> + if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
> + return -EINVAL;
[...]
