Re: [PATCH 2/4] perf record: Enable off-cpu analysis with BPF

Arnaldo Carvalho de Melo
2022-05-10T17:03:03Z

Em Fri, May 06, 2022 at 01:16:25PM -0700, Namhyung Kim escreveu:
> Add --off-cpu option to enable the off-cpu profiling with BPF. It'd
> use a bpf_output event and rename it to "offcpu-time". Samples will
> be synthesized at the end of the record session using data from a BPF
> map which contains the aggregated off-cpu time at context switches.
> So it needs root privilege to get the off-cpu profiling.
>
> Each sample will have a separate user stacktrace so it will skip
> kernel threads. The sample ip will be set from the stacktrace and
> other sample data will be updated accordingly. Currently it only
> handles some basic sample types.
>
> The sample timestamp is set to a dummy value just not to bother with
> other events during the sorting. So it has a very big initial value
> and increase it on processing each samples.
>
> Good thing is that it can be used together with regular profiling like
> cpu cycles. If you don't want to that, you can use a dummy event to
> enable off-cpu profiling only.
>
> Example output:
> $ sudo perf record --off-cpu perf bench sched messaging -l 1000
>
> $ sudo perf report --stdio --call-graph=no
> # Total Lost Samples: 0
> #
> # Samples: 41K of event 'cycles'
> # Event count (approx.): 42137343851
> ...
>
> # Samples: 1K of event 'offcpu-time'
> # Event count (approx.): 587990831640
> #
> # Children Self Command Shared Object Symbol
> # ........ ........ ............... .................. .........................
> #
> 81.66% 0.00% sched-messaging libc-2.33.so [.] __libc_start_main
> 81.66% 0.00% sched-messaging perf [.] cmd_bench
> 81.66% 0.00% sched-messaging perf [.] main
> 81.66% 0.00% sched-messaging perf [.] run_builtin
> 81.43% 0.00% sched-messaging perf [.] bench_sched_messaging
> 40.86% 40.86% sched-messaging libpthread-2.33.so [.] __read
> 37.66% 37.66% sched-messaging libpthread-2.33.so [.] __write
> 2.91% 2.91% sched-messaging libc-2.33.so [.] __poll
> ...
>
> As you can see it spent most of off-cpu time in read and write in
> bench_sched_messaging(). The --call-graph=no was added just to make
> the output concise here.
>
> It uses perf hooks facility to control BPF program during the record
> session rather than adding new BPF/off-cpu specific calls.
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
> tools/perf/Documentation/perf-record.txt | 10 ++
> tools/perf/Makefile.perf | 1 +
> tools/perf/builtin-record.c | 21 +++
> tools/perf/util/Build | 1 +
> tools/perf/util/bpf_off_cpu.c | 208 +++++++++++++++++++++++
> tools/perf/util/bpf_skel/off_cpu.bpf.c | 139 +++++++++++++++
> tools/perf/util/off_cpu.h | 22 +++
> 7 files changed, 402 insertions(+)
> create mode 100644 tools/perf/util/bpf_off_cpu.c
> create mode 100644 tools/perf/util/bpf_skel/off_cpu.bpf.c
> create mode 100644 tools/perf/util/off_cpu.h
>
> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
> index 465be4e62a17..8084e3fb73a1 100644
> --- a/tools/perf/Documentation/perf-record.txt
> +++ b/tools/perf/Documentation/perf-record.txt
> @@ -758,6 +758,16 @@ include::intel-hybrid.txt[]
> If the URLs is not specified, the value of DEBUGINFOD_URLS
> system environment variable is used.
>
> +--off-cpu::
> + Enable off-cpu profiling with BPF. The BPF program will collect
> + task scheduling information with (user) stacktrace and save them
> + as sample data of a software event named "offcpu-time". The
> + sample period will have the time the task slept in nano-second.

nanoseconds.
> +
> + Note that BPF can collect stacktrace using frame pointer ("fp")

stack traces

> + only, as of now. So the applications built without the frame
> + pointer might see bogus addresses.

One possible improvement is to check if debugging info is available in
the traced program and check if -fnoomit-frame-pointer is present:

⬢[acme@toolbox perf]$ readelf -wi ~/bin/perf | grep DW_AT_producer -m1
<d> DW_AT_producer : (indirect string, offset: 0x6627): GNU C99 11.2.1 20220127 (Red Hat 11.2.1-9) -mtune=generic -march=x86-64 -ggdb3 -O6 -std=gnu99 -fno-omit-frame-pointer -funwind-tables -fstack-protector-all
⬢[acme@toolbox perf]$

If debugging info is available and -fno-omit-frame-pointer isn't in the
DW_AT_producer string, then we can print a flashing warning and even
don't use the stack traces.

That or some other more robust method to detect that frame pointers are
valid.

Some more comments below.

> +
> SEE ALSO
> --------
> linkperf:perf-stat[1], linkperf:perf-list[1], linkperf:perf-intel-pt[1]
> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index 6e5aded855cc..8f738e11356d 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -1038,6 +1038,7 @@ SKEL_TMP_OUT := $(abspath $(SKEL_OUT)/.tmp)
> SKELETONS := $(SKEL_OUT)/bpf_prog_profiler.skel.h
> SKELETONS += $(SKEL_OUT)/bperf_leader.skel.h $(SKEL_OUT)/bperf_follower.skel.h
> SKELETONS += $(SKEL_OUT)/bperf_cgroup.skel.h $(SKEL_OUT)/func_latency.skel.h
> +SKELETONS += $(SKEL_OUT)/off_cpu.skel.h
>
> $(SKEL_TMP_OUT) $(LIBBPF_OUTPUT):
> $(Q)$(MKDIR) -p $@
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index a5cf6a99d67f..4e0285ca9a2d 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -49,6 +49,7 @@
> #include "util/clockid.h"
> #include "util/pmu-hybrid.h"
> #include "util/evlist-hybrid.h"
> +#include "util/off_cpu.h"
> #include "asm/bug.h"
> #include "perf.h"
> #include "cputopo.h"
> @@ -162,6 +163,7 @@ struct record {
> bool buildid_mmap;
> bool timestamp_filename;
> bool timestamp_boundary;
> + bool off_cpu;
> struct switch_output switch_output;
> unsigned long long samples;
> unsigned long output_max_size; /* = 0: unlimited */
> @@ -903,6 +905,11 @@ static int record__config_text_poke(struct evlist *evlist)
> return 0;
> }
>
> +static int record__config_off_cpu(struct record *rec)
> +{
> + return off_cpu_prepare(rec->evlist);
> +}
> +
> static bool record__kcore_readable(struct machine *machine)
> {
> char kcore[PATH_MAX];
> @@ -2600,6 +2607,9 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> } else
> status = err;
>
> + if (rec->off_cpu)
> + rec->bytes_written += off_cpu_write(rec->session);
> +
> record__synthesize(rec, true);
> /* this will be recalculated during process_buildids() */
> rec->samples = 0;
> @@ -3324,6 +3334,9 @@ static struct option __record_options[] = {
> OPT_CALLBACK_OPTARG(0, "threads", &record.opts, NULL, "spec",
> "write collected trace data into several data files using parallel threads",
> record__parse_threads),
> +#ifdef HAVE_BPF_SKEL
> + OPT_BOOLEAN(0, "off-cpu", &record.off_cpu, "Enable off-cpu analysis"),
> +#endif

Since this is in the documentation, I think we should have an #else
clause and state that --off-cpu needs building with some specific make
command line.

> OPT_END()
> };
>
> @@ -3981,6 +3994,14 @@ int cmd_record(int argc, const char **argv)
> }
> }
>
> + if (rec->off_cpu) {
> + err = record__config_off_cpu(rec);
> + if (err) {
> + pr_err("record__config_off_cpu failed, error %d\n", err);
> + goto out;
> + }
> + }
> +
> if (record_opts__config(&rec->opts)) {
> err = -EINVAL;
> goto out;
> diff --git a/tools/perf/util/Build b/tools/perf/util/Build
> index 9a7209a99e16..a51267d88ca9 100644
> --- a/tools/perf/util/Build
> +++ b/tools/perf/util/Build
> @@ -147,6 +147,7 @@ perf-$(CONFIG_LIBBPF) += bpf_map.o
> perf-$(CONFIG_PERF_BPF_SKEL) += bpf_counter.o
> perf-$(CONFIG_PERF_BPF_SKEL) += bpf_counter_cgroup.o
> perf-$(CONFIG_PERF_BPF_SKEL) += bpf_ftrace.o
> +perf-$(CONFIG_PERF_BPF_SKEL) += bpf_off_cpu.o
> perf-$(CONFIG_BPF_PROLOGUE) += bpf-prologue.o
> perf-$(CONFIG_LIBELF) += symbol-elf.o
> perf-$(CONFIG_LIBELF) += probe-file.o
> diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
> new file mode 100644
> index 000000000000..1f87d2a9b86d
> --- /dev/null
> +++ b/tools/perf/util/bpf_off_cpu.c
> @@ -0,0 +1,208 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include "util/bpf_counter.h"
> +#include "util/debug.h"
> +#include "util/evsel.h"
> +#include "util/evlist.h"
> +#include "util/off_cpu.h"
> +#include "util/perf-hooks.h"
> +#include "util/session.h"
> +#include <bpf/bpf.h>
> +
> +#include "bpf_skel/off_cpu.skel.h"
> +
> +#define MAX_STACKS 32
> +/* we don't need actual timestamp, just want to put the samples at last */
> +#define OFF_CPU_TIMESTAMP (~0ull << 32)
> +
> +static struct off_cpu_bpf *skel;
> +
> +struct off_cpu_key {
> + u32 pid;
> + u32 tgid;
> + u32 stack_id;
> + u32 state;
> + u64 cgroup_id;
> +};
> +
> +union off_cpu_data {
> + struct perf_event_header hdr;
> + u64 array[1024 / sizeof(u64)];
> +};
> +
> +static int off_cpu_config(struct evlist *evlist)
> +{
> + struct evsel *evsel;
> + struct perf_event_attr attr = {
> + .type = PERF_TYPE_SOFTWARE,
> + .config = PERF_COUNT_SW_BPF_OUTPUT,
> + .size = sizeof(attr), /* to capture ABI version */
> + };
> +
> + evsel = evsel__new(&attr);
> + if (!evsel)
> + return -ENOMEM;
> +
> + evsel->core.attr.freq = 1;
> + evsel->core.attr.sample_period = 1;
> + /* off-cpu analysis depends on stack trace */
> + evsel->core.attr.sample_type = PERF_SAMPLE_CALLCHAIN;
> +
> + evlist__add(evlist, evsel);
> +
> + free(evsel->name);
> + evsel->name = strdup("offcpu-time");
> + if (evsel->name == NULL)
> + return -ENOMEM;

But at this point the evsel was left in the evlist? Shouldn't we start
it all by trying to allocate this string and don't bother to call
evsel__new() on failure, etc?

> +
> + return 0;
> +}
> +
> +static void off_cpu_start(void *arg __maybe_unused)
> +{
> + skel->bss->enabled = 1;
> +}
> +
> +static void off_cpu_finish(void *arg __maybe_unused)
> +{
> + skel->bss->enabled = 0;
> + off_cpu_bpf__destroy(skel);
> +}
> +
> +int off_cpu_prepare(struct evlist *evlist)
> +{
> + int err;
> +
> + if (off_cpu_config(evlist) < 0) {
> + pr_err("Failed to config off-cpu BPF event\n");
> + return -1;
> + }
> +
> + set_max_rlimit();
> +
> + skel = off_cpu_bpf__open_and_load();
> + if (!skel) {
> + pr_err("Failed to open off-cpu skeleton\n");

"BPF sleketon", to give a bit more context to users?

> + return -1;
> + }
> +
> + err = off_cpu_bpf__attach(skel);
> + if (err) {
> + pr_err("Failed to attach off-cpu skeleton\n");

Ditto.

> + goto out;
> + }
> +
> + if (perf_hooks__set_hook("record_start", off_cpu_start, NULL) ||
> + perf_hooks__set_hook("record_end", off_cpu_finish, NULL)) {
> + pr_err("Failed to attach off-cpu skeleton\n");
> + goto out;
> + }
> +
> + return 0;
> +
> +out:
> + off_cpu_bpf__destroy(skel);
> + return -1;
> +}
> +
> +int off_cpu_write(struct perf_session *session)
> +{
> + int bytes = 0, size;
> + int fd, stack;
> + bool found = false;
> + u64 sample_type, val, sid = 0;
> + struct evsel *evsel;
> + struct perf_data_file *file = &session->data->file;
> + struct off_cpu_key prev, key;
> + union off_cpu_data data = {
> + .hdr = {
> + .type = PERF_RECORD_SAMPLE,
> + .misc = PERF_RECORD_MISC_USER,
> + },
> + };
> + u64 tstamp = OFF_CPU_TIMESTAMP;
> +
> + skel->bss->enabled = 0;
> +
> + evlist__for_each_entry(session->evlist, evsel) {
> + if (!strcmp(evsel__name(evsel), "offcpu-time")) {
> + found = true;
> + break;
> + }
> + }

Don't we have some evlist__find_evsel function?

struct evsel *evlist__find_evsel_by_str(struct evlist *evlist, const char *str)
{
struct evsel *evsel;

evlist__for_each_entry(evlist, evsel) {
if (!evsel->name)
continue;
if (strcmp(str, evsel->name) == 0)
return evsel;
}

return NULL;
}

> +
> + if (!found) {
> + pr_err("offcpu-time evsel not found\n");
> + return 0;
> + }
> +
> + sample_type = evsel->core.attr.sample_type;
> +
> + if (sample_type & (PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER)) {
> + if (evsel->core.id)
> + sid = evsel->core.id[0];
> + }
> +
> + fd = bpf_map__fd(skel->maps.off_cpu);
> + stack = bpf_map__fd(skel->maps.stacks);
> + memset(&prev, 0, sizeof(prev));
> +
> + while (!bpf_map_get_next_key(fd, &prev, &key)) {
> + int n = 1; /* start from perf_event_header */
> + int ip_pos = -1;
> +
> + bpf_map_lookup_elem(fd, &key, &val);
> +
> + if (sample_type & PERF_SAMPLE_IDENTIFIER)
> + data.array[n++] = sid;
> + if (sample_type & PERF_SAMPLE_IP) {
> + ip_pos = n;
> + data.array[n++] = 0; /* will be updated */
> + }
> + if (sample_type & PERF_SAMPLE_TID)
> + data.array[n++] = (u64)key.pid << 32 | key.tgid;
> + if (sample_type & PERF_SAMPLE_TIME)
> + data.array[n++] = tstamp;
> + if (sample_type & PERF_SAMPLE_ID)
> + data.array[n++] = sid;
> + if (sample_type & PERF_SAMPLE_CPU)
> + data.array[n++] = 0;
> + if (sample_type & PERF_SAMPLE_PERIOD)
> + data.array[n++] = val;
> + if (sample_type & PERF_SAMPLE_CALLCHAIN) {
> + int len = 0;
> +
> + /* data.array[n] is callchain->nr (updated later) */
> + data.array[n + 1] = PERF_CONTEXT_USER;
> + data.array[n + 2] = 0;
> +
> + bpf_map_lookup_elem(stack, &key.stack_id, &data.array[n + 2]);
> + while (data.array[n + 2 + len])
> + len++;
> +
> + /* update length of callchain */
> + data.array[n] = len + 1;
> +
> + /* update sample ip with the first callchain entry */
> + if (ip_pos >= 0)
> + data.array[ip_pos] = data.array[n + 2];
> +
> + /* calculate sample callchain data array length */
> + n += len + 2;
> + }
> + /* TODO: handle more sample types */
> +
> + size = n * sizeof(u64);
> + data.hdr.size = size;
> + bytes += size;
> +
> + if (perf_data_file__write(file, &data, size) < 0) {
> + pr_err("failed to write perf data, error: %m\n");
> + return bytes;
> + }
> +
> + prev = key;
> + /* increase dummy timestamp to sort later samples */
> + tstamp++;
> + }
> + return bytes;
> +}
> diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> new file mode 100644
> index 000000000000..5173ed882fdf
> --- /dev/null
> +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> @@ -0,0 +1,139 @@
> +// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +// Copyright (c) 2022 Google
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include <bpf/bpf_core_read.h>
> +
> +/* task->flags for off-cpu analysis */
> +#define PF_KTHREAD 0x00200000 /* I am a kernel thread */
> +
> +/* task->state for off-cpu analysis */
> +#define TASK_INTERRUPTIBLE 0x0001
> +#define TASK_UNINTERRUPTIBLE 0x0002
> +
> +#define MAX_STACKS 32
> +#define MAX_ENTRIES 102400
> +
> +struct tstamp_data {
> + __u32 stack_id;
> + __u32 state;
> + __u64 timestamp;
> +};
> +
> +struct offcpu_key {
> + __u32 pid;
> + __u32 tgid;
> + __u32 stack_id;
> + __u32 state;
> +};
> +
> +struct {
> + __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> + __uint(key_size, sizeof(__u32));
> + __uint(value_size, MAX_STACKS * sizeof(__u64));
> + __uint(max_entries, MAX_ENTRIES);
> +} stacks SEC(".maps");
> +
> +struct {
> + __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
> + __uint(map_flags, BPF_F_NO_PREALLOC);
> + __type(key, int);
> + __type(value, struct tstamp_data);
> +} tstamp SEC(".maps");
> +
> +struct {
> + __uint(type, BPF_MAP_TYPE_HASH);
> + __uint(key_size, sizeof(struct offcpu_key));
> + __uint(value_size, sizeof(__u64));
> + __uint(max_entries, MAX_ENTRIES);
> +} off_cpu SEC(".maps");
> +
> +/* old kernel task_struct definition */
> +struct task_struct___old {
> + long state;
> +} __attribute__((preserve_access_index));
> +
> +int enabled = 0;
> +
> +/*
> + * Old kernel used to call it task_struct->state and now it's '__state'.
> + * Use BPF CO-RE "ignored suffix rule" to deal with it like below:
> + *
> + * https://nakryiko.com/posts/bpf-core-reference-guide/#handling-incompatible-field-and-type-changes
> + */
> +static inline int get_task_state(struct task_struct *t)
> +{
> + if (bpf_core_field_exists(t->__state))
> + return BPF_CORE_READ(t, __state);
> +
> + /* recast pointer to capture task_struct___old type for compiler */
> + struct task_struct___old *t_old = (void *)t;
> +
> + /* now use old "state" name of the field */
> + return BPF_CORE_READ(t_old, state);
> +}
> +
> +SEC("tp_btf/sched_switch")
> +int on_switch(u64 *ctx)
> +{
> + __u64 ts;
> + int state;
> + __u32 stack_id;
> + struct task_struct *prev, *next;
> + struct tstamp_data *pelem;
> +
> + if (!enabled)
> + return 0;
> +
> + prev = (struct task_struct *)ctx[1];
> + next = (struct task_struct *)ctx[2];
> + state = get_task_state(prev);
> +
> + ts = bpf_ktime_get_ns();
> +
> + if (prev->flags & PF_KTHREAD)
> + goto next;
> + if (state != TASK_INTERRUPTIBLE &&
> + state != TASK_UNINTERRUPTIBLE)
> + goto next;
> +
> + stack_id = bpf_get_stackid(ctx, &stacks,
> + BPF_F_FAST_STACK_CMP | BPF_F_USER_STACK);
> +
> + pelem = bpf_task_storage_get(&tstamp, prev, NULL,
> + BPF_LOCAL_STORAGE_GET_F_CREATE);
> + if (!pelem)
> + goto next;
> +
> + pelem->timestamp = ts;
> + pelem->state = state;
> + pelem->stack_id = stack_id;
> +
> +next:
> + pelem = bpf_task_storage_get(&tstamp, next, NULL, 0);
> +
> + if (pelem && pelem->timestamp) {
> + struct offcpu_key key = {
> + .pid = next->pid,
> + .tgid = next->tgid,
> + .stack_id = pelem->stack_id,
> + .state = pelem->state,
> + };
> + __u64 delta = ts - pelem->timestamp;
> + __u64 *total;
> +
> + total = bpf_map_lookup_elem(&off_cpu, &key);
> + if (total)
> + *total += delta;
> + else
> + bpf_map_update_elem(&off_cpu, &key, &delta, BPF_ANY);
> +
> + /* prevent to reuse the timestamp later */
> + pelem->timestamp = 0;
> + }
> +
> + return 0;
> +}
> +
> +char LICENSE[] SEC("license") = "Dual BSD/GPL";
> diff --git a/tools/perf/util/off_cpu.h b/tools/perf/util/off_cpu.h
> new file mode 100644
> index 000000000000..4113bd19a2e4
> --- /dev/null
> +++ b/tools/perf/util/off_cpu.h
> @@ -0,0 +1,22 @@
> +#ifndef PERF_UTIL_OFF_CPU_H
> +#define PERF_UTIL_OFF_CPU_H
> +
> +struct evlist;
> +struct perf_session;
> +
> +#ifdef HAVE_BPF_SKEL
> +int off_cpu_prepare(struct evlist *evlist);
> +int off_cpu_write(struct perf_session *session);
> +#else
> +static inline int off_cpu_prepare(struct evlist *evlist __maybe_unused)
> +{
> + return -1;
> +}
> +
> +static inline int off_cpu_write(struct perf_session *session __maybe_unused)
> +{
> + return -1;
> +}
> +#endif
> +
> +#endif /* PERF_UTIL_OFF_CPU_H */
> --
> 2.36.0.512.ge40c2bad7a-goog

--

- Arnaldo