On 26 May 2020, at 7:29, Andrii Nakryiko wrote:

On Mon, May 25, 2020 at 2:01 PM Eelco Chaudron [off-list ref] 
wrote:

quoted

This new API, perf_buffer__consume, can be used as follows:

I wonder, was it inspired by yet-to-be committed
ring_buffer__consume() or it's just a coincidence?

Just coincidence, I was needing a function to flush the remaining ring 
entries, as I was using a larger wakeup_events value.
Initially, I called the function ring_buffer_flush(), but once I noticed 
your patch I renamed it :)

quoted

- When you have a perf ring where wakeup_events is higher than 1,
  and you have remaining data in the rings you would like to pull
  out on exit (or maybe based on a timeout).
- For low latency cases where you burn a CPU that constantly polls
  the queues.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
---
 tools/lib/bpf/libbpf.c   |   23 +++++++++++++++++++++++
 tools/lib/bpf/libbpf.h   |    1 +
 tools/lib/bpf/libbpf.map |    1 +
 3 files changed, 25 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index fa04cbe547ed..cbef3dac7507 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -8456,6 +8456,29 @@ int perf_buffer__poll(struct perf_buffer *pb, 

int timeout_ms)
        return cnt < 0 ? -errno : cnt;
 }

+int perf_buffer__consume(struct perf_buffer *pb)
+{
+       int i;
+
+       if (!pb)
+               return -EINVAL;

we don't check this in perf_buffer__poll, IMO, checking this in every
"method" is an overkill.

Ack, will fix in v2

quoted

+
+       if (!pb->cpu_bufs)
+               return 0;

no need to check. It's either non-NULL for valid perf_buffer, or
calloc could return NULL if pb->cpu_cnt is zero (not sure it's
possible, but still), but then loop below will never access
pb->cpu_bufs[i].

Agreed, was just adding some safety checks, but in the constantly poll 
mode this is a lot of overhead. Will remover in v2.

quoted

+
+       for (i = 0; i < pb->cpu_cnt && pb->cpu_bufs[i]; i++) {

I think pb->cpu_bufs[i] check is wrong, it will stop iteration
prematurely if cpu_bufs are sparsely populated. So move check inside
and continue loop if NULL.

Mimicked the behavior from other functions, however just to be safe I 
split it up.

quoted

+               int err;

nit: declare it together with "i" above, similar to how
perf_buffer__poll does it

Put it down here as it’s only used in the context of the for loop, but 
will move it up in the v2.

quoted

+               struct perf_cpu_buf *cpu_buf = pb->cpu_bufs[i];
+
+               err = perf_buffer__process_records(pb, cpu_buf);
+               if (err) {
+                       pr_warn("error while processing records: 
%d\n", err);
+                       return err;
+               }
+       }
+       return 0;
+}
+
 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 */

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 8ea69558f0a8..1e2e399a5f2c 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h

@@ -533,6 +533,7 @@ perf_buffer__new_raw(int map_fd, size_t page_cnt,

 LIBBPF_API void perf_buffer__free(struct perf_buffer *pb);
 LIBBPF_API int perf_buffer__poll(struct perf_buffer *pb, int 

timeout_ms);
+LIBBPF_API int perf_buffer__consume(struct perf_buffer *pb);

 typedef enum bpf_perf_event_ret
        (*bpf_perf_event_print_t)(struct perf_event_header *hdr,

diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 0133d469d30b..381a7342ecfc 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map

@@ -262,4 +262,5 @@ LIBBPF_0.0.9 {
                bpf_link_get_fd_by_id;
                bpf_link_get_next_id;
                bpf_program__attach_iter;
+               perf_buffer__consume;
 } LIBBPF_0.0.8;

Thanks for the review, will send out a v2 soon.