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Message-ID: <CAP-5=fXenfuYZ-ArxDjt7OY7Z+KHJ8fL+ewOUo+9Uno0E=k0pw@mail.gmail.com>
Date: Wed, 18 May 2022 20:58:25 -0700
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Andi Kleen <ak@...ux.intel.com>,
Song Liu <songliubraving@...com>, Hao Luo <haoluo@...gle.com>,
Milian Wolff <milian.wolff@...b.com>, bpf@...r.kernel.org,
linux-perf-users@...r.kernel.org,
Blake Jones <blakejones@...gle.com>
Subject: Re: [PATCH 2/6] perf record: Enable off-cpu analysis with BPF
On Wed, May 18, 2022 at 3:47 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> 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@...nel.org>
Acked-by: Ian Rogers <irogers@...gle.com>
> ---
> tools/perf/Documentation/perf-record.txt | 10 ++
> tools/perf/Makefile.perf | 1 +
> tools/perf/builtin-record.c | 25 +++
> tools/perf/util/Build | 1 +
> tools/perf/util/bpf_off_cpu.c | 204 +++++++++++++++++++++++
> tools/perf/util/bpf_skel/off_cpu.bpf.c | 139 +++++++++++++++
> tools/perf/util/off_cpu.h | 24 +++
> 7 files changed, 404 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..b4e9ef7edfef 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 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]
> 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..91f88501412e 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 +3334,7 @@ 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),
> + OPT_BOOLEAN(0, "off-cpu", &record.off_cpu, "Enable off-cpu analysis"),
> OPT_END()
> };
>
> @@ -3743,6 +3754,12 @@ int cmd_record(int argc, const char **argv)
> set_nobuild('\0', "vmlinux", true);
> # undef set_nobuild
> # 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;
> @@ -3981,6 +3998,14 @@ 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..9ed7aca3f4ac
> --- /dev/null
> +++ b/tools/perf/util/bpf_off_cpu.c
> @@ -0,0 +1,204 @@
> +// 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)) {
An off-topic thought here. I was looking at tool events and thinking
that rather than have global state and the counter reading getting
those global values, it would be nice if the tool events had private
state that was created on open, freed on close and then did the
appropriate thing for enable/read/disable. If we were object oriented
I was thinking tool events could be a subclass of evsel that would
override the appropriate functions. Something that strikes me as silly
is that tool events have a dummy event file descriptor from
perf_event_open, in part because the evsel code paths are shared.
Anyway, if we made evsels have something like a virtual method table,
we could do similar tricks with off-cpu and BPF created events.
Possibly this could lead to better structured code and more reuse.
Thanks,
Ian
> + 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;
> +}
> 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..375d03c424ea
> --- /dev/null
> +++ b/tools/perf/util/off_cpu.h
> @@ -0,0 +1,24 @@
> +#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 */
> --
> 2.36.1.124.g0e6072fb45-goog
>
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