Re: [PATCH v3 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

[Yonghong Song <yhs@fb.com>]

On 1/23/20 1:23 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@dxuuu.xyz>
> ---
>   include/uapi/linux/bpf.h | 15 ++++++++++++++-
>   kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++++++
>   2 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f1d74a2bd234..50c580c8a201 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2892,6 +2892,18 @@ union bpf_attr {
>    *		Obtain the 64bit jiffies
>    *	Return
>    *		The 64 bit jiffies
> + *
> + * int bpf_perf_prog_read_branches(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size)
> + *	Description
> + *		For en eBPF program attached to a perf event, retrieve the

en => an

> + *		branch records (struct perf_branch_entry) associated to *ctx*
> + *		and store it in	the buffer pointed by *buf* up to size
> + *		*buf_size* bytes.
> + *
> + *		Any unused parts of *buf* will be filled with zeros.
> + *	Return
> + *		On success, number of bytes written to *buf*. On error, a
> + *		negative value.
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -3012,7 +3024,8 @@ union bpf_attr {
>   	FN(probe_read_kernel_str),	\
>   	FN(tcp_send_ack),		\
>   	FN(send_signal_thread),		\
> -	FN(jiffies64),
> +	FN(jiffies64),			\
> +	FN(perf_prog_read_branches),
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>    * function eBPF program intends to call
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 19e793aa441a..24c51272a1f7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1028,6 +1028,35 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
>            .arg3_type      = ARG_CONST_SIZE,
>   };
>   
> +BPF_CALL_3(bpf_perf_prog_read_branches, struct bpf_perf_event_data_kern *, ctx,
> +	   void *, buf, u32, size)
> +{
> +	struct perf_branch_stack *br_stack = ctx->data->br_stack;
> +	u32 to_copy = 0, to_clear = size;
> +	int err = -EINVAL;
> +
> +	if (unlikely(!br_stack))
> +		goto clear;
> +
> +	to_copy = min_t(u32, br_stack->nr * sizeof(struct perf_branch_entry), size);
> +	to_clear -= to_copy;
> +
> +	memcpy(buf, br_stack->entries, to_copy);
> +	err = to_copy;
> +clear:
> +	memset(buf + to_copy, 0, to_clear);
> +	return err;

If size < u32, br_stack->nr * sizeof(struct perf_branch_entry),
user has no way to know whether some entries are not copied except
repeated trying larger buffers until the return value is smaller
than input buffer size.

I think returning the expected buffer size to users should be a good 
thing? We may not have malloc today in bpf, but future malloc thing 
should help in this case.

In user space, user may have a fixed buffer, repeated `read` should
read all values.

Using bpf_probe_read(), repeated read with adjusted source pointer
can also read all buffers.

One possible design is to add a flag to the function, e.g., if
flag == GET_BR_STACK_NR, return br_stack->nr in buf/size.
if flag == GET_BR_STACK, return br_stack->entries in buf/size.

What do you think?


> +}
> +
> +static const struct bpf_func_proto bpf_perf_prog_read_branches_proto = {
> +         .func           = bpf_perf_prog_read_branches,
> +         .gpl_only       = true,
> +         .ret_type       = RET_INTEGER,
> +         .arg1_type      = ARG_PTR_TO_CTX,
> +         .arg2_type      = ARG_PTR_TO_UNINIT_MEM,
> +         .arg3_type      = ARG_CONST_SIZE,
> +};
> +
>   static const struct bpf_func_proto *
>   pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   {
> @@ -1040,6 +1069,8 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   		return &bpf_get_stack_proto_tp;
>   	case BPF_FUNC_perf_prog_read_value:
>   		return &bpf_perf_prog_read_value_proto;
> +	case BPF_FUNC_perf_prog_read_branches:
> +		return &bpf_perf_prog_read_branches_proto;
>   	default:
>   		return tracing_func_proto(func_id, prog);
>   	}
>