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From: Namhyung Kim <namhyung@kernel.org>
Date: Tue, 27 Feb 2024 16:31:12 -0800
Message-ID: <CAM9d7chRWk2mpph3zVTQXJeaWaFTreUZAWvHafoYqx5vsfpTpw@mail.gmail.com>
Subject: Re: [PATCH RESEND] perf lock contention: Account contending locks too
To: Ian Rogers <irogers@google.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>, Jiri Olsa <jolsa@kernel.org>, 
	Adrian Hunter <adrian.hunter@intel.com>, Peter Zijlstra <peterz@infradead.org>, 
	Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>, 
	linux-perf-users@vger.kernel.org, Song Liu <song@kernel.org>, bpf@vger.kernel.org
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On Tue, Feb 27, 2024 at 3:49=E2=80=AFPM Ian Rogers <irogers@google.com> wro=
te:
>
> On Fri, Feb 9, 2024 at 3:07=E2=80=AFPM Namhyung Kim <namhyung@kernel.org>=
 wrote:
> >
> > Currently it accounts the contention using delta between timestamps in
> > lock:contention_begin and lock:contention_end tracepoints.  But it mean=
s
> > the lock should see the both events during the monitoring period.
> >
> > Actually there are 4 cases that happen with the monitoring:
> >
> >                 monitoring period
> >             /                       \
> >             |                       |
> >  1:  B------+-----------------------+--------E
> >  2:    B----+-------------E         |
> >  3:         |           B-----------+----E
> >  4:         |     B-------------E   |
> >             |                       |
> >             t0                      t1
> >
> > where B and E mean contention BEGIN and END, respectively.  So it only
> > accounts the case 4 for now.  It seems there's no way to handle the cas=
e
> > 1.  The case 2 might be handled if it saved the timestamp (t0), but it
> > lacks the information from the B notably the flags which shows the lock
> > types.  Also it could be a nested lock which it currently ignores.  So
> > I think we should ignore the case 2.
> >
> > However we can handle the case 3 if we save the timestamp (t1) at the
> > end of the period.  And then it can iterate the map entries in the
> > userspace and update the lock stat accordinly.
> >
> > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > ---
> >  tools/perf/util/bpf_lock_contention.c         | 116 ++++++++++++++++++
> >  .../perf/util/bpf_skel/lock_contention.bpf.c  |  16 +--
> >  tools/perf/util/bpf_skel/lock_data.h          |   7 ++
> >  3 files changed, 132 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bp=
f_lock_contention.c
> > index 31ff19afc20c..d6bafd9a3955 100644
> > --- a/tools/perf/util/bpf_lock_contention.c
> > +++ b/tools/perf/util/bpf_lock_contention.c
> > @@ -179,6 +179,119 @@ int lock_contention_prepare(struct lock_contentio=
n *con)
> >         return 0;
> >  }
> >
> > +static void mark_end_timestamp(void)
> > +{
> > +       DECLARE_LIBBPF_OPTS(bpf_test_run_opts, opts,
> > +               .flags =3D BPF_F_TEST_RUN_ON_CPU,
>
> It seems strange that this and the raw tracepoint are both test. I see
> similar non-test uses in libbpf-tools. It would be worth documenting
> that this isn't test code. Everything else LGTM.

It's a BPF syscall API that allows to run a certain kind of BPF program
directly and not to necessarily be in a test.

Thanks,
Namhyung

>
> > +       );
> > +       int prog_fd =3D bpf_program__fd(skel->progs.end_timestamp);
> > +
> > +       bpf_prog_test_run_opts(prog_fd, &opts);
> > +}
> > +
> > +static void update_lock_stat(int map_fd, int pid, u64 end_ts,
> > +                            enum lock_aggr_mode aggr_mode,
> > +                            struct tstamp_data *ts_data)
> > +{
> > +       u64 delta;
> > +       struct contention_key stat_key =3D {};
> > +       struct contention_data stat_data;
> > +
> > +       if (ts_data->timestamp >=3D end_ts)
> > +               return;
> > +
> > +       delta =3D end_ts - ts_data->timestamp;
> > +
> > +       switch (aggr_mode) {
> > +       case LOCK_AGGR_CALLER:
> > +               stat_key.stack_id =3D ts_data->stack_id;
> > +               break;
> > +       case LOCK_AGGR_TASK:
> > +               stat_key.pid =3D pid;
> > +               break;
> > +       case LOCK_AGGR_ADDR:
> > +               stat_key.lock_addr_or_cgroup =3D ts_data->lock;
> > +               break;
> > +       case LOCK_AGGR_CGROUP:
> > +               /* TODO */
> > +               return;
> > +       default:
> > +               return;
> > +       }
> > +
> > +       if (bpf_map_lookup_elem(map_fd, &stat_key, &stat_data) < 0)
> > +               return;
> > +
> > +       stat_data.total_time +=3D delta;
> > +       stat_data.count++;
> > +
> > +       if (delta > stat_data.max_time)
> > +               stat_data.max_time =3D delta;
> > +       if (delta < stat_data.min_time)
> > +               stat_data.min_time =3D delta;
> > +
> > +       bpf_map_update_elem(map_fd, &stat_key, &stat_data, BPF_EXIST);
> > +}
> > +
> > +/*
> > + * Account entries in the tstamp map (which didn't see the correspondi=
ng
> > + * lock:contention_end tracepoint) using end_ts.
> > + */
> > +static void account_end_timestamp(struct lock_contention *con)
> > +{
> > +       int ts_fd, stat_fd;
> > +       int *prev_key, key;
> > +       u64 end_ts =3D skel->bss->end_ts;
> > +       int total_cpus;
> > +       enum lock_aggr_mode aggr_mode =3D con->aggr_mode;
> > +       struct tstamp_data ts_data, *cpu_data;
> > +
> > +       /* Iterate per-task tstamp map (key =3D TID) */
> > +       ts_fd =3D bpf_map__fd(skel->maps.tstamp);
> > +       stat_fd =3D bpf_map__fd(skel->maps.lock_stat);
> > +
> > +       prev_key =3D NULL;
> > +       while (!bpf_map_get_next_key(ts_fd, prev_key, &key)) {
> > +               if (bpf_map_lookup_elem(ts_fd, &key, &ts_data) =3D=3D 0=
) {
> > +                       int pid =3D key;
> > +
> > +                       if (aggr_mode =3D=3D LOCK_AGGR_TASK && con->own=
er)
> > +                               pid =3D ts_data.flags;
> > +
> > +                       update_lock_stat(stat_fd, pid, end_ts, aggr_mod=
e,
> > +                                        &ts_data);
> > +               }
> > +
> > +               prev_key =3D &key;
> > +       }
> > +
> > +       /* Now it'll check per-cpu tstamp map which doesn't have TID. *=
/
> > +       if (aggr_mode =3D=3D LOCK_AGGR_TASK || aggr_mode =3D=3D LOCK_AG=
GR_CGROUP)
> > +               return;
> > +
> > +       total_cpus =3D cpu__max_cpu().cpu;
> > +       ts_fd =3D bpf_map__fd(skel->maps.tstamp_cpu);
> > +
> > +       cpu_data =3D calloc(total_cpus, sizeof(*cpu_data));
> > +       if (cpu_data =3D=3D NULL)
> > +               return;
> > +
> > +       prev_key =3D NULL;
> > +       while (!bpf_map_get_next_key(ts_fd, prev_key, &key)) {
> > +               if (bpf_map_lookup_elem(ts_fd, &key, cpu_data) < 0)
> > +                       goto next;
> > +
> > +               for (int i =3D 0; i < total_cpus; i++) {
> > +                       update_lock_stat(stat_fd, -1, end_ts, aggr_mode=
,
> > +                                        &cpu_data[i]);
> > +               }
> > +
> > +next:
> > +               prev_key =3D &key;
> > +       }
> > +       free(cpu_data);
> > +}
> > +
> >  int lock_contention_start(void)
> >  {
> >         skel->bss->enabled =3D 1;
> > @@ -188,6 +301,7 @@ int lock_contention_start(void)
> >  int lock_contention_stop(void)
> >  {
> >         skel->bss->enabled =3D 0;
> > +       mark_end_timestamp();
> >         return 0;
> >  }
> >
> > @@ -301,6 +415,8 @@ int lock_contention_read(struct lock_contention *co=
n)
> >         if (stack_trace =3D=3D NULL)
> >                 return -1;
> >
> > +       account_end_timestamp(con);
> > +
> >         if (con->aggr_mode =3D=3D LOCK_AGGR_TASK) {
> >                 struct thread *idle =3D __machine__findnew_thread(machi=
ne,
> >                                                                 /*pid=
=3D*/0,
> > diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/per=
f/util/bpf_skel/lock_contention.bpf.c
> > index 95cd8414f6ef..fb54bd38e7d0 100644
> > --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > @@ -19,13 +19,6 @@
> >  #define LCB_F_PERCPU   (1U << 4)
> >  #define LCB_F_MUTEX    (1U << 5)
> >
> > -struct tstamp_data {
> > -       __u64 timestamp;
> > -       __u64 lock;
> > -       __u32 flags;
> > -       __s32 stack_id;
> > -};
> > -
> >  /* callstack storage  */
> >  struct {
> >         __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> > @@ -140,6 +133,8 @@ int perf_subsys_id =3D -1;
> >  /* determine the key of lock stat */
> >  int aggr_mode;
> >
> > +__u64 end_ts;
> > +
> >  /* error stat */
> >  int task_fail;
> >  int stack_fail;
> > @@ -559,4 +554,11 @@ int BPF_PROG(collect_lock_syms)
> >         return 0;
> >  }
> >
> > +SEC("raw_tp/bpf_test_finish")
> > +int BPF_PROG(end_timestamp)
> > +{
> > +       end_ts =3D bpf_ktime_get_ns();
> > +       return 0;
> > +}
> > +
> >  char LICENSE[] SEC("license") =3D "Dual BSD/GPL";
> > diff --git a/tools/perf/util/bpf_skel/lock_data.h b/tools/perf/util/bpf=
_skel/lock_data.h
> > index 08482daf61be..36af11faad03 100644
> > --- a/tools/perf/util/bpf_skel/lock_data.h
> > +++ b/tools/perf/util/bpf_skel/lock_data.h
> > @@ -3,6 +3,13 @@
> >  #ifndef UTIL_BPF_SKEL_LOCK_DATA_H
> >  #define UTIL_BPF_SKEL_LOCK_DATA_H
> >
> > +struct tstamp_data {
> > +       u64 timestamp;
> > +       u64 lock;
> > +       u32 flags;
> > +       u32 stack_id;
> > +};
> > +
> >  struct contention_key {
> >         u32 stack_id;
> >         u32 pid;
> > --
> > 2.43.0.687.g38aa6559b0-goog
> >