Re: [PATCH RESEND] perf lock contention: Account contending locks too

From: Namhyung Kim
Date: Tue Feb 27 2024 - 19:31:39 EST

On Tue, Feb 27, 2024 at 3:49 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Fri, Feb 9, 2024 at 3:07 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > Currently it accounts the contention using delta between timestamps in
> > lock:contention_begin and lock:contention_end tracepoints. But it means
> > 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 case
> > 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@xxxxxxxxxx>
> > ---
> > 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/bpf_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_contention *con)
> > return 0;
> > }
> >
> > +static void mark_end_timestamp(void)
> > +{
> > + DECLARE_LIBBPF_OPTS(bpf_test_run_opts, opts,
> > + .flags = 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 = 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 = {};
> > + struct contention_data stat_data;
> > +
> > + if (ts_data->timestamp >= end_ts)
> > + return;
> > +
> > + delta = end_ts - ts_data->timestamp;
> > +
> > + switch (aggr_mode) {
> > + case LOCK_AGGR_CALLER:
> > + stat_key.stack_id = ts_data->stack_id;
> > + break;
> > + case LOCK_AGGR_TASK:
> > + stat_key.pid = pid;
> > + break;
> > + case LOCK_AGGR_ADDR:
> > + stat_key.lock_addr_or_cgroup = 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 += delta;
> > + stat_data.count++;
> > +
> > + if (delta > stat_data.max_time)
> > + stat_data.max_time = delta;
> > + if (delta < stat_data.min_time)
> > + stat_data.min_time = delta;
> > +
> > + bpf_map_update_elem(map_fd, &stat_key, &stat_data, BPF_EXIST);
> > +}
> > +
> > +/*
> > + * Account entries in the tstamp map (which didn't see the corresponding
> > + * 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 = skel->bss->end_ts;
> > + int total_cpus;
> > + enum lock_aggr_mode aggr_mode = con->aggr_mode;
> > + struct tstamp_data ts_data, *cpu_data;
> > +
> > + /* Iterate per-task tstamp map (key = TID) */
> > + ts_fd = bpf_map__fd(skel->maps.tstamp);
> > + stat_fd = bpf_map__fd(skel->maps.lock_stat);
> > +
> > + prev_key = NULL;
> > + while (!bpf_map_get_next_key(ts_fd, prev_key, &key)) {
> > + if (bpf_map_lookup_elem(ts_fd, &key, &ts_data) == 0) {
> > + int pid = key;
> > +
> > + if (aggr_mode == LOCK_AGGR_TASK && con->owner)
> > + pid = ts_data.flags;
> > +
> > + update_lock_stat(stat_fd, pid, end_ts, aggr_mode,
> > + &ts_data);
> > + }
> > +
> > + prev_key = &key;
> > + }
> > +
> > + /* Now it'll check per-cpu tstamp map which doesn't have TID. */
> > + if (aggr_mode == LOCK_AGGR_TASK || aggr_mode == LOCK_AGGR_CGROUP)
> > + return;
> > +
> > + total_cpus = cpu__max_cpu().cpu;
> > + ts_fd = bpf_map__fd(skel->maps.tstamp_cpu);
> > +
> > + cpu_data = calloc(total_cpus, sizeof(*cpu_data));
> > + if (cpu_data == NULL)
> > + return;
> > +
> > + prev_key = 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 = 0; i < total_cpus; i++) {
> > + update_lock_stat(stat_fd, -1, end_ts, aggr_mode,
> > + &cpu_data[i]);
> > + }
> > +
> > +next:
> > + prev_key = &key;
> > + }
> > + free(cpu_data);
> > +}
> > +
> > int lock_contention_start(void)
> > {
> > skel->bss->enabled = 1;
> > @@ -188,6 +301,7 @@ int lock_contention_start(void)
> > int lock_contention_stop(void)
> > {
> > skel->bss->enabled = 0;
> > + mark_end_timestamp();
> > return 0;
> > }
> >
> > @@ -301,6 +415,8 @@ int lock_contention_read(struct lock_contention *con)
> > if (stack_trace == NULL)
> > return -1;
> >
> > + account_end_timestamp(con);
> > +
> > if (con->aggr_mode == LOCK_AGGR_TASK) {
> > struct thread *idle = __machine__findnew_thread(machine,
> > /*pid=*/0,
> > diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/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 = -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 = bpf_ktime_get_ns();
> > + return 0;
> > +}
> > +
> > char LICENSE[] SEC("license") = "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
> >