Re: [PATCH v5 bpf-next 1/2] bpf: Add bpf_read_branch_records() helper

[Yonghong Song]

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;
[...]