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Message-ID: <Zd8lkcb5irCOY4-m@x1>
Date: Wed, 28 Feb 2024 09:22:41 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Adrian Hunter <adrian.hunter@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
linux-perf-users@...r.kernel.org, Song Liu <song@...nel.org>,
bpf@...r.kernel.org
Subject: Re: [PATCH v2] perf lock contention: Account contending locks too
On Tue, Feb 27, 2024 at 09:33:35PM -0800, Namhyung Kim 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.
Perhaps have a separate output listing locks that were found to be with
at least tE - t0 time, with perhaps a backtrace at that END time?
With that we wouldn't miss that info, however incomplete it is and the
user would try running again, perhaps for a longer time, or start
monitoring before the observed workload starts, etc.
Anyway:
Reviwed-by: Arnaldo Carvalho de Melo <acme@...hat.com>
- Arnaldo
> 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@...nel.org>
> ---
> v2: add a comment on mark_end_timestamp (Ian)
>
> tools/perf/util/bpf_lock_contention.c | 120 ++++++++++++++++++
> .../perf/util/bpf_skel/lock_contention.bpf.c | 16 ++-
> tools/perf/util/bpf_skel/lock_data.h | 7 +
> 3 files changed, 136 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> index 31ff19afc20c..9af76c6b2543 100644
> --- a/tools/perf/util/bpf_lock_contention.c
> +++ b/tools/perf/util/bpf_lock_contention.c
> @@ -179,6 +179,123 @@ int lock_contention_prepare(struct lock_contention *con)
> return 0;
> }
>
> +/*
> + * Run the BPF program directly using BPF_PROG_TEST_RUN to update the end
> + * timestamp in ktime so that it can calculate delta easily.
> + */
> +static void mark_end_timestamp(void)
> +{
> + DECLARE_LIBBPF_OPTS(bpf_test_run_opts, opts,
> + .flags = BPF_F_TEST_RUN_ON_CPU,
> + );
> + 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 +305,7 @@ int lock_contention_start(void)
> int lock_contention_stop(void)
> {
> skel->bss->enabled = 0;
> + mark_end_timestamp();
> return 0;
> }
>
> @@ -301,6 +419,8 @@ 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;
> --
> 2.44.0.rc1.240.g4c46232300-goog
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