perf record: Enable off-cpu analysis with BPF

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 <[e-mail]>
Acked-by: Ian Rogers <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Blake Jones <[e-mail]>
Cc: Hao Luo <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Milian Wolff <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-record.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 nanoseconds.
+
+	Note that BPF can collect stack traces using frame pointer ("fp")
+	only, as of now.  So the applications built without the frame
+	pointer might see bogus addresses.
+
 SEE ALSO
 --------
 linkperf:perf-stat[1], linkperf:perf-list[1], linkperf:perf-intel-pt[1]

tools/perf/Makefile.perf

···
 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 $@

tools/perf/builtin-record.c

···
 #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"
···
 	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 */
···
 	evsel__set_sample_bit(evsel, TIME);
 
 	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)
···
 	} 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;
···
 	OPT_CALLBACK_OPTARG(0, "threads", &record.opts, NULL, "spec",
 			    "write collected trace data into several data files using parallel threads",
 			    record__parse_threads),
+	OPT_BOOLEAN(0, "off-cpu", &record.off_cpu, "Enable off-cpu analysis"),
 	OPT_END()
 };
 
···
 # undef REASON
 #endif
 
+#ifndef HAVE_BPF_SKEL
+# define set_nobuild(s, l, m, c) set_option_nobuild(record_options, s, l, m, c)
+	set_nobuild('\0', "off-cpu", "no BUILD_BPF_SKEL=1", true);
+# undef set_nobuild
+#endif
+
 	rec->opts.affinity = PERF_AFFINITY_SYS;
 
 	rec->evlist = evlist__new();
···
 		err = record__config_text_poke(rec->evlist);
 		if (err) {
 			pr_err("record__config_text_poke failed, error %d\n", err);
+			goto out;
+		}
+	}
+
+	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;
 		}
 	}

tools/perf/util/Build

···
 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

tools/perf/util/bpf_off_cpu.c

···
+// 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;
+};
+
+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 */
+	};
+	char *evname = strdup(OFFCPU_EVENT);
+
+	if (evname == NULL)
+		return -ENOMEM;
+
+	evsel = evsel__new(&attr);
+	if (!evsel) {
+		free(evname);
+		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 = evname;
+
+	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 BPF skeleton\n");
+		return -1;
+	}
+
+	err = off_cpu_bpf__attach(skel);
+	if (err) {
+		pr_err("Failed to attach off-cpu BPF skeleton\n");
+		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;
+	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;
+
+	evsel = evlist__find_evsel_by_str(session->evlist, OFFCPU_EVENT);
+	if (evsel == NULL) {
+		pr_err("%s evsel not found\n", OFFCPU_EVENT);
+		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;
+}

tools/perf/util/bpf_skel/off_cpu.bpf.c

···
+// 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";

tools/perf/util/off_cpu.h

···
+#ifndef PERF_UTIL_OFF_CPU_H
+#define PERF_UTIL_OFF_CPU_H
+
+struct evlist;
+struct perf_session;
+
+#define OFFCPU_EVENT  "offcpu-time"
+
+#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 */