Re: [PATCH 4/7] perf record: Record dropped sample count

From: Ian Rogers
Date: Tue Feb 14 2023 - 11:41:32 EST

On Mon, Feb 13, 2023 at 9:05 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> When it uses bpf filters, event might drop some samples. It'd be nice
> if it can report how many samples it lost. As LOST_SAMPLES event can
> carry the similar information, let's use it for bpf filters.
>
> To indicate it's from BPF filters, add a new misc flag for that and
> do not display cpu load warnings.

Can you potentially have lost samples from being too slow to drain the
ring buffer and dropped samples because of BPF? Is it possible to
distinguish lost and dropped with this approach?

Thanks,
Ian

> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> tools/perf/builtin-record.c | 37 ++++++++++++++++++++++--------------
> tools/perf/util/bpf-filter.c | 7 +++++++
> tools/perf/util/bpf-filter.h | 5 +++++
> tools/perf/util/session.c | 3 ++-
> 4 files changed, 37 insertions(+), 15 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index c81047a78f3e..3201d1a1ea1f 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1869,24 +1869,16 @@ record__switch_output(struct record *rec, bool at_exit)
> return fd;
> }
>
> -static void __record__read_lost_samples(struct record *rec, struct evsel *evsel,
> +static void __record__save_lost_samples(struct record *rec, struct evsel *evsel,
> struct perf_record_lost_samples *lost,
> - int cpu_idx, int thread_idx)
> + int cpu_idx, int thread_idx, u64 lost_count,
> + u16 misc_flag)
> {
> - struct perf_counts_values count;
> struct perf_sample_id *sid;
> struct perf_sample sample = {};
> int id_hdr_size;
>
> - if (perf_evsel__read(&evsel->core, cpu_idx, thread_idx, &count) < 0) {
> - pr_err("read LOST count failed\n");
> - return;
> - }
> -
> - if (count.lost == 0)
> - return;
> -
> - lost->lost = count.lost;
> + lost->lost = lost_count;
> if (evsel->core.ids) {
> sid = xyarray__entry(evsel->core.sample_id, cpu_idx, thread_idx);
> sample.id = sid->id;
> @@ -1895,6 +1887,7 @@ static void __record__read_lost_samples(struct record *rec, struct evsel *evsel,
> id_hdr_size = perf_event__synthesize_id_sample((void *)(lost + 1),
> evsel->core.attr.sample_type, &sample);
> lost->header.size = sizeof(*lost) + id_hdr_size;
> + lost->header.misc = misc_flag;
> record__write(rec, NULL, lost, lost->header.size);
> }
>
> @@ -1918,6 +1911,7 @@ static void record__read_lost_samples(struct record *rec)
>
> evlist__for_each_entry(session->evlist, evsel) {
> struct xyarray *xy = evsel->core.sample_id;
> + u64 lost_count;
>
> if (xy == NULL || evsel->core.fd == NULL)
> continue;
> @@ -1929,12 +1923,27 @@ static void record__read_lost_samples(struct record *rec)
>
> for (int x = 0; x < xyarray__max_x(xy); x++) {
> for (int y = 0; y < xyarray__max_y(xy); y++) {
> - __record__read_lost_samples(rec, evsel, lost, x, y);
> + struct perf_counts_values count;
> +
> + if (perf_evsel__read(&evsel->core, x, y, &count) < 0) {
> + pr_err("read LOST count failed\n");
> + goto out;
> + }
> +
> + if (count.lost) {
> + __record__save_lost_samples(rec, evsel, lost,
> + x, y, count.lost, 0);
> + }
> }
> }
> +
> + lost_count = perf_bpf_filter__lost_count(evsel);
> + if (lost_count)
> + __record__save_lost_samples(rec, evsel, lost, 0, 0, lost_count,
> + PERF_RECORD_MISC_LOST_SAMPLES_BPF);
> }
> +out:
> free(lost);
> -
> }
>
> static volatile sig_atomic_t workload_exec_errno;
> diff --git a/tools/perf/util/bpf-filter.c b/tools/perf/util/bpf-filter.c
> index f47420cf81c9..11fb391c92e9 100644
> --- a/tools/perf/util/bpf-filter.c
> +++ b/tools/perf/util/bpf-filter.c
> @@ -76,6 +76,13 @@ int perf_bpf_filter__destroy(struct evsel *evsel)
> return 0;
> }
>
> +u64 perf_bpf_filter__lost_count(struct evsel *evsel)
> +{
> + struct sample_filter_bpf *skel = evsel->bpf_skel;
> +
> + return skel ? skel->bss->dropped : 0;
> +}
> +
> struct perf_bpf_filter_expr *perf_bpf_filter_expr__new(unsigned long sample_flags,
> enum perf_bpf_filter_op op,
> unsigned long val)
> diff --git a/tools/perf/util/bpf-filter.h b/tools/perf/util/bpf-filter.h
> index 6077930073f9..36b44c8188ab 100644
> --- a/tools/perf/util/bpf-filter.h
> +++ b/tools/perf/util/bpf-filter.h
> @@ -22,6 +22,7 @@ struct perf_bpf_filter_expr *perf_bpf_filter_expr__new(unsigned long sample_flag
> int perf_bpf_filter__parse(struct list_head *expr_head, const char *str);
> int perf_bpf_filter__prepare(struct evsel *evsel);
> int perf_bpf_filter__destroy(struct evsel *evsel);
> +u64 perf_bpf_filter__lost_count(struct evsel *evsel);
>
> #else /* !HAVE_BPF_SKEL */
>
> @@ -38,5 +39,9 @@ static inline int perf_bpf_filter__destroy(struct evsel *evsel)
> {
> return -ENOSYS;
> }
> +static inline u64 perf_bpf_filter__lost_count(struct evsel *evsel)
> +{
> + return 0;
> +}
> #endif /* HAVE_BPF_SKEL*/
> #endif /* PERF_UTIL_BPF_FILTER_H */
> \ No newline at end of file
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index 749d5b5c135b..7d8d057d1772 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -1582,7 +1582,8 @@ static int machines__deliver_event(struct machines *machines,
> evlist->stats.total_lost += event->lost.lost;
> return tool->lost(tool, event, sample, machine);
> case PERF_RECORD_LOST_SAMPLES:
> - if (tool->lost_samples == perf_event__process_lost_samples)
> + if (tool->lost_samples == perf_event__process_lost_samples &&
> + !(event->header.misc & PERF_RECORD_MISC_LOST_SAMPLES_BPF))
> evlist->stats.total_lost_samples += event->lost_samples.lost;
> return tool->lost_samples(tool, event, sample, machine);
> case PERF_RECORD_READ:
> --
> 2.39.1.581.gbfd45094c4-goog
>