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Date:   Wed, 20 Jan 2021 09:37:27 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     open list <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Jiri Olsa <jolsa@redhat.com>, Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v7 3/3] perf-stat: enable counting events for BPF programs
Message-ID: <20210120123727.GR12699@kernel.org>
References: <20201229214214.3413833-1-songliubraving@fb.com>
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 <20210118193817.GG12699@kernel.org>
 <379919CC-594F-40C5-A10E-97E048F73AE2@fb.com>
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Em Tue, Jan 19, 2021 at 07:30:21PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Jan 19, 2021 at 09:54:50PM +0000, Song Liu escreveu:
> > 
> > 
> > > On Jan 19, 2021, at 8:31 AM, Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> > > 
> > > Em Tue, Jan 19, 2021 at 11:42:49AM -0300, Arnaldo Carvalho de Melo escreveu:
> > >> Em Tue, Jan 19, 2021 at 11:31:44AM -0300, Arnaldo Carvalho de Melo escreveu:
> > >>> Em Tue, Jan 19, 2021 at 12:48:19AM +0000, Song Liu escreveu:
> > >>>>> On Jan 18, 2021, at 11:38 AM, Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> > >>>> We are looking at two issues:
> > >>>> 1. Cannot recursively attach;
> > >>>> 2. prog FD 3 doesn't have valid btf. 
> > >> 
> > >>>> #1 was caused by the verifier disallowing attaching fentry/fexit program 
> > >>>> to program with type BPF_PROG_TYPE_TRACING (in bpf_check_attach_target). 
> > >>>> This constraint was added when we only had fentry/fexit in the TRACING
> > >>>> type. We have extended the TRACING type to many other use cases, like 
> > >>>> "tp_btf/", "fmod_ret" and "iter/". Therefore, it is good time to revisit 
> > >>>> this constraint. I will work on this. 
> > >> 
> > >>>> For #2, we require the target program to have BTF. I guess we won't remove
> > >>>> this requirement.
> > >> 
> > >>>> While I work on improving #1, could you please test with some kprobe 
> > >>>> programs? For example, we can use fileslower.py from bcc. 
> > >> 
> > >>> Sure, and please consider improving the error messages to state what you
> > >>> described above.
> > >> 
> > >> Terminal 1:
> > >> 
> > >> [root@five perf]# perf trace -e 5sec.c
> > >> ^C
> > >> [root@five perf]# cat 5sec.c
> > >> #include <bpf.h>
> > >> 
> > >> #define NSEC_PER_SEC	1000000000L
> > >> 
> > >> int probe(hrtimer_nanosleep, rqtp)(void *ctx, int err, long long sec)
> > >> {
> > >> 	return sec / NSEC_PER_SEC == 5;
> > >> }
> > >> 
> > >> license(GPL);
> > >> [root@five perf]# perf trace -e 5sec.c/max-stack=16/
> > >>     0.000 sleep/3739435 perf_bpf_probe:hrtimer_nanosleep(__probe_ip: -1743337312, rqtp: 5000000000)
> > >>                                       hrtimer_nanosleep ([kernel.kallsyms])
> > >>                                       common_nsleep ([kernel.kallsyms])
> > >>                                       __x64_sys_clock_nanosleep ([kernel.kallsyms])
> > >>                                       do_syscall_64 ([kernel.kallsyms])
> > >>                                       entry_SYSCALL_64_after_hwframe ([kernel.kallsyms])
> > >>                                       __clock_nanosleep_2 (/usr/lib64/libc-2.32.so)
> > >> 
> > >> 
> > >> Terminal 2:
> > >> 
> > >> [root@five ~]# perf stat -e cycles -b 180 -I 1000
> > >> libbpf: elf: skipping unrecognized data section(9) .eh_frame
> > >> libbpf: elf: skipping relo section(15) .rel.eh_frame for section(9) .eh_frame
> > >> perf: util/bpf_counter.c:227: bpf_program_profiler__read: Assertion `skel != NULL' failed.
> > >> Aborted (core dumped)
> > > 
> > > Out to lunch, will continue later, but this may help you figure this out
> > > till then :)
> > > 
> > > Starting program: /root/bin/perf stat -e cycles -b 244 -I 1000
> > > [Thread debugging using libthread_db enabled]
> > > Using host libthread_db library "/lib64/libthread_db.so.1".
> > > 
> > > Breakpoint 1, bpf_program_profiler_load_one (evsel=0xce02c0, prog_id=244) at util/bpf_counter.c:96
> > > 96	{
> > > (gdb) n
> > > 104		prog_fd = bpf_prog_get_fd_by_id(prog_id);
> > > (gdb) 
> > > 105		if (prog_fd < 0) {
> > > (gdb) 
> > > 109		counter = bpf_counter_alloc();
> > > (gdb) 
> > > 110		if (!counter) {
> > > (gdb) n
> > > 115		skel = bpf_prog_profiler_bpf__open();
> > > (gdb) p counter
> > > $9 = (struct bpf_counter *) 0xce09e0
> > > (gdb) p *counter
> > > $10 = {skel = 0x0, list = {next = 0xce09e8, prev = 0xce09e8}}
> > > (gdb) p *counter
> > > $11 = {skel = 0x0, list = {next = 0xce09e8, prev = 0xce09e8}}
> > > (gdb) n
> > > libbpf: elf: skipping unrecognized data section(9) .eh_frame
> > > libbpf: elf: skipping relo section(15) .rel.eh_frame for section(9) .eh_frame
> > > 116		if (!skel) {
> > > (gdb) 
> > > 121		skel->rodata->num_cpu = evsel__nr_cpus(evsel);
> > > (gdb) 
> > > 123		bpf_map__resize(skel->maps.events, evsel__nr_cpus(evsel));
> > > (gdb) 
> > > 124		bpf_map__resize(skel->maps.fentry_readings, 1);
> > > (gdb) 
> > > 125		bpf_map__resize(skel->maps.accum_readings, 1);
> > > (gdb) 
> > > 127		prog_name = bpf_target_prog_name(prog_fd);
> > > (gdb) 
> > > 128		if (!prog_name) {
> > > (gdb) 
> > > 133		bpf_object__for_each_program(prog, skel->obj) {
> > > (gdb) 
> > > 134			err = bpf_program__set_attach_target(prog, prog_fd, prog_name);
> > > (gdb) 
> > > 135			if (err) {
> > > (gdb) 
> > > 133		bpf_object__for_each_program(prog, skel->obj) {
> > > (gdb) p evsel
> > > $12 = (struct evsel *) 0xce02c0
> > > (gdb) p evsel->name
> > > $13 = 0xce04e0 "cycles"
> > > (gdb) n
> > > 134			err = bpf_program__set_attach_target(prog, prog_fd, prog_name);
> > > (gdb) 
> > > 135			if (err) {
> > > (gdb) 
> > > 133		bpf_object__for_each_program(prog, skel->obj) {
> > > (gdb) 
> > > 141		set_max_rlimit();
> > > (gdb) 
> > > 142		err = bpf_prog_profiler_bpf__load(skel);
> > > (gdb) 
> > > 143		if (err) {
> > > (gdb) 
> > > 148		assert(skel != NULL);
> > > (gdb) 
> > > 149		counter->skel = skel;
> > > (gdb) 
> > > 150		list_add(&counter->list, &evsel->bpf_counter_list);
> > > (gdb) c
> > > Continuing.
> > > 
> > > Breakpoint 4, bpf_program_profiler__install_pe (evsel=0xce02c0, cpu=0, fd=3) at util/bpf_counter.c:247
> > > 247	{
> > > (gdb) n
> > > 252		list_for_each_entry(counter, &evsel->bpf_counter_list, list) {
> > > (gdb) 
> > > 253			skel = counter->skel;
> > > (gdb) watch counter->skel
> > > Hardware watchpoint 6: counter->skel
> > > (gdb) p counter->skel
> > > $14 = (void *) 0xce0a00
> > > (gdb) n
> > > 254			assert(skel != NULL);
> > > (gdb) p skel
> > > $15 = (struct bpf_prog_profiler_bpf *) 0xce0a00
> > > (gdb) c
> > > Continuing.
> > > 
> > > Hardware watchpoint 6: counter->skel
> > > 
> > > Old value = (void *) 0xce0a00
> > > New value = (void *) 0x0
> > > 0x00000000005cf45e in bpf_program_profiler__install_pe (evsel=0xce02c0, cpu=0, fd=3) at util/bpf_counter.c:252
> > > 252		list_for_each_entry(counter, &evsel->bpf_counter_list, list) {
> > 
> > So it was the list operation that set counter->skel to NULL? I am really confused...
> 
> Yep, I'm confused as well and trying to reproduce this, but got
> sidetracked...

Coming back to this, now it is exploding later:

Breakpoint 8, bpf_program_profiler__install_pe (evsel=0xce02c0, cpu=22, fd=32) at util/bpf_counter.c:254
254			assert(skel != NULL);
(gdb) p skel
$52 = (struct bpf_prog_profiler_bpf *) 0xce0a00
(gdb) c
Continuing.

Breakpoint 4, bpf_program_profiler__install_pe (evsel=0xce02c0, cpu=23, fd=33) at util/bpf_counter.c:247
247	{
(gdb) p skel
$53 = (struct bpf_prog_profiler_bpf *) 0xce04c0
(gdb) c
Continuing.

Breakpoint 8, bpf_program_profiler__install_pe (evsel=0xce02c0, cpu=23, fd=33) at util/bpf_counter.c:254
254			assert(skel != NULL);
(gdb) p skel
$54 = (struct bpf_prog_profiler_bpf *) 0xce0a00
(gdb) c
Continuing.

Breakpoint 2, bpf_program_profiler__enable (evsel=0xce02c0) at util/bpf_counter.c:192
192	{
(gdb) n
196		list_for_each_entry(counter, &evsel->bpf_counter_list, list) {
(gdb)
197			assert(counter->skel != NULL);
(gdb)
198			ret = bpf_prog_profiler_bpf__attach(counter->skel);
(gdb) c
Continuing.

Breakpoint 3, bpf_program_profiler__read (evsel=0xce02c0) at util/bpf_counter.c:208
208	{
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x00000000005cf34b in bpf_program_profiler__read (evsel=0x0) at util/bpf_counter.c:224
224		list_for_each_entry(counter, &evsel->bpf_counter_list, list) {
(gdb) p evsel
$55 = (struct evsel *) 0x0
(gdb) bt
#0  0x00000000005cf34b in bpf_program_profiler__read (evsel=0x0) at util/bpf_counter.c:224
#1  0x0000000000000000 in ?? ()
(gdb) bt
#0  0x00000000005cf34b in bpf_program_profiler__read (evsel=0x0) at util/bpf_counter.c:224
#1  0x0000000000000000 in ?? ()
(gdb)
(gdb) info threads
  Id   Target Id                                  Frame
* 1    Thread 0x7ffff647f900 (LWP 1725711) "perf" 0x00000000005cf34b in bpf_program_profiler__read (evsel=0x0) at util/bpf_counter.c:224
(gdb)