Re: [PATCH v3 20/50] perf record: Be lazier in allocating lost samples buffer

[Ian Rogers]

On Tue, Oct 24, 2023 at 8:44 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
> Hello,
>
> On 2023/10/25 6:23, Ian Rogers wrote:
> > Wait until a lost sample occurs to allocate the lost samples buffer,
> > often the buffer isn't necessary. This saves a 64kb allocation and
> > 5.3kb of peak memory consumption.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >   tools/perf/builtin-record.c | 29 +++++++++++++++++++----------
> >   1 file changed, 19 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 9b4f3805ca92..b6c8c1371b39 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -1924,21 +1924,13 @@ static void __record__save_lost_samples(struct record *rec, struct evsel *evsel,
> >   static void record__read_lost_samples(struct record *rec)
> >   {
> >       struct perf_session *session = rec->session;
> > -     struct perf_record_lost_samples *lost;
> > +     struct perf_record_lost_samples *lost = NULL;
> >       struct evsel *evsel;
> >
> >       /* there was an error during record__open */
> >       if (session->evlist == NULL)
> >               return;
> >
> > -     lost = zalloc(PERF_SAMPLE_MAX_SIZE);
> > -     if (lost == NULL) {
> > -             pr_debug("Memory allocation failed\n");
> > -             return;
> > -     }
> > -
> > -     lost->header.type = PERF_RECORD_LOST_SAMPLES;
> > -
> >       evlist__for_each_entry(session->evlist, evsel) {
> >               struct xyarray *xy = evsel->core.sample_id;
> >               u64 lost_count;
> > @@ -1961,6 +1953,14 @@ static void record__read_lost_samples(struct record *rec)
> >                               }
> >
> >                               if (count.lost) {
> > +                                     if (!lost) {
> > +                                             lost = zalloc(PERF_SAMPLE_MAX_SIZE);
> > +                                             if (!lost) {
> > +                                                     pr_debug("Memory allocation failed\n");
> > +                                                     return;
> > +                                             }
> > +                                             lost->header.type = PERF_RECORD_LOST_SAMPLES;
> > +                                     }
> >                                       __record__save_lost_samples(rec, evsel, lost,
> >                                                                   x, y, count.lost, 0);
> >                               }
> > @@ -1968,9 +1968,18 @@ static void record__read_lost_samples(struct record *rec)
> >               }
> >
> >               lost_count = perf_bpf_filter__lost_count(evsel);
> > -             if (lost_count)
> > +             if (lost_count) {
> > +                     if (!lost) {
> > +                             lost = zalloc(PERF_SAMPLE_MAX_SIZE);
> > +                             if (!lost) {
> > +                                     pr_debug("Memory allocation failed\n");
> > +                                     return;
> > +                             }
> > +                             lost->header.type = PERF_RECORD_LOST_SAMPLES;
> > +                     }
> >                       __record__save_lost_samples(rec, evsel, lost, 0, 0, lost_count,
> >                                                   PERF_RECORD_MISC_LOST_SAMPLES_BPF);
> > +             }
> >       }
>
> Can zalloc for `lost` be moved to __record__save_lost_samples?
> This simplifies the code.

Hmm.. seems marginal. This change makes the code not return in
record__read_lost_samples when the memory allocation fails, so I'm
50/50 on it.

Thanks,
Ian

>
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index dcf288a4fb9a..8d2eb746031a 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1888,14 +1888,25 @@ record__switch_output(struct record *rec, bool
> at_exit)
>   }
>
>   static void __record__save_lost_samples(struct record *rec, struct
> evsel *evsel,
> -                                       struct perf_record_lost_samples
> *lost,
> +                                       struct perf_record_lost_samples
> **plost,
>                                          int cpu_idx, int thread_idx,
> u64 lost_count,
>                                          u16 misc_flag)
>   {
>          struct perf_sample_id *sid;
>          struct perf_sample sample = {};
> +       struct perf_record_lost_samples *lost = *plost;
>          int id_hdr_size;
>
> +       if (!lost) {
> +               lost = zalloc(PERF_SAMPLE_MAX_SIZE);
> +               if (!lost) {
> +                       pr_debug("Memory allocation failed\n");
> +                       return;
> +               }
> +               lost->header.type = PERF_RECORD_LOST_SAMPLES;
> +               *plost = lost;
> +       }
> +
>          lost->lost = lost_count;
>          if (evsel->core.ids) {
>                  sid = xyarray__entry(evsel->core.sample_id, cpu_idx,
> thread_idx);
> @@ -1912,21 +1923,13 @@ static void __record__save_lost_samples(struct
> record *rec, struct evsel *evsel,
>   static void record__read_lost_samples(struct record *rec)
>   {
>          struct perf_session *session = rec->session;
> -       struct perf_record_lost_samples *lost;
> +       struct perf_record_lost_samples *lost = NULL;
>          struct evsel *evsel;
>
>          /* there was an error during record__open */
>          if (session->evlist == NULL)
>                  return;
>
> -       lost = zalloc(PERF_SAMPLE_MAX_SIZE);
> -       if (lost == NULL) {
> -               pr_debug("Memory allocation failed\n");
> -               return;
> -       }
> -
> -       lost->header.type = PERF_RECORD_LOST_SAMPLES;
> -
>          evlist__for_each_entry(session->evlist, evsel) {
>                  struct xyarray *xy = evsel->core.sample_id;
>                  u64 lost_count;
> @@ -1949,7 +1952,7 @@ static void record__read_lost_samples(struct
> record *rec)
>                                  }
>
>                                  if (count.lost) {
> -                                       __record__save_lost_samples(rec,
> evsel, lost,
> +                                       __record__save_lost_samples(rec,
> evsel, &lost,
>                                                                      x,
> y, count.lost, 0);
>                                  }
>                          }
> @@ -1957,11 +1960,12 @@ static void record__read_lost_samples(struct
> record *rec)
>
>                  lost_count = perf_bpf_filter__lost_count(evsel);
>                  if (lost_count)
> -                       __record__save_lost_samples(rec, evsel, lost, 0,
> 0, lost_count,
> +                       __record__save_lost_samples(rec, evsel, &lost,
> 0, 0, lost_count,
>
> PERF_RECORD_MISC_LOST_SAMPLES_BPF);
>          }
>   out:
> -       free(lost);
> +       if (lost)
> +               free(lost);
>   }
>
>   static volatile sig_atomic_t workload_exec_errno;
>
>
> Thanks,
> Yang
>