netdev - Re: [PATCH v5 bpf-next 1/5] libbpf: add perf buffer API

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Date:   Sat, 6 Jul 2019 05:42:33 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH v5 bpf-next 1/5] libbpf: add perf buffer API



On 7/5/19 9:35 PM, Andrii Nakryiko wrote:
> BPF_MAP_TYPE_PERF_EVENT_ARRAY map is often used to send data from BPF program
> to user space for additional processing. libbpf already has very low-level API
> to read single CPU perf buffer, bpf_perf_event_read_simple(), but it's hard to
> use and requires a lot of code to set everything up. This patch adds
> perf_buffer abstraction on top of it, abstracting setting up and polling
> per-CPU logic into simple and convenient API, similar to what BCC provides.
> 
> perf_buffer__new() sets up per-CPU ring buffers and updates corresponding BPF
> map entries. It accepts two user-provided callbacks: one for handling raw
> samples and one for get notifications of lost samples due to buffer overflow.
> 
> perf_buffer__new_raw() is similar, but provides more control over how
> perf events are set up (by accepting user-provided perf_event_attr), how
> they are handled (perf_event_header pointer is passed directly to
> user-provided callback), and on which CPUs ring buffers are created
> (it's possible to provide a list of CPUs and corresponding map keys to
> update). This API allows advanced users fuller control.
> 
> perf_buffer__poll() is used to fetch ring buffer data across all CPUs,
> utilizing epoll instance.
> 
> perf_buffer__free() does corresponding clean up and unsets FDs from BPF map.
> 
> All APIs are not thread-safe. User should ensure proper locking/coordination if
> used in multi-threaded set up.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
>   tools/lib/bpf/libbpf.c   | 366 +++++++++++++++++++++++++++++++++++++++
>   tools/lib/bpf/libbpf.h   |  49 ++++++
>   tools/lib/bpf/libbpf.map |   4 +
>   3 files changed, 419 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 2a08eb106221..72149d68b8c1 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -32,7 +32,9 @@
>   #include <linux/limits.h>
>   #include <linux/perf_event.h>
>   #include <linux/ring_buffer.h>
> +#include <sys/epoll.h>
>   #include <sys/ioctl.h>
> +#include <sys/mman.h>
>   #include <sys/stat.h>
>   #include <sys/types.h>
>   #include <sys/vfs.h>
> @@ -4354,6 +4356,370 @@ bpf_perf_event_read_simple(void *mmap_mem, size_t mmap_size, size_t page_size,
>   	return ret;
>   }
>   
> +struct perf_buffer;
> +
> +struct perf_buffer_params {
> +	struct perf_event_attr *attr;
> +	/* if event_cb is specified, it takes precendence */
> +	perf_buffer_event_fn event_cb;
> +	/* sample_cb and lost_cb are higher-level common-case callbacks */
> +	perf_buffer_sample_fn sample_cb;
> +	perf_buffer_lost_fn lost_cb;
> +	void *ctx;
> +	int cpu_cnt;
> +	int *cpus;
[...]
> +
> +int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms)
> +{
> +	int cnt, err;
> +
> +	cnt = epoll_wait(pb->epoll_fd, pb->events, pb->cpu_cnt, timeout_ms);
> +	for (int i = 0; i < cnt; i++) {

Find one compilation error here.

libbpf.c: In function ‘perf_buffer__poll’:
libbpf.c:4728:2: error: ‘for’ loop initial declarations are only allowed 
in C99 mode
   for (int i = 0; i < cnt; i++) {
   ^

> +		struct perf_cpu_buf *cpu_buf = pb->events[i].data.ptr;
> +
> +		err = perf_buffer__process_records(pb, cpu_buf);
> +		if (err) {
> +			pr_warning("error while processing records: %d\n", err);
> +			return err;
> +		}
> +	}
> +	return cnt < 0 ? -errno : cnt;
> +}
> +
>   struct bpf_prog_info_array_desc {
>   	int	array_offset;	/* e.g. offset of jited_prog_insns */
>   	int	count_offset;	/* e.g. offset of jited_prog_len */
[...]