libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind #4463

ekyooo · 2023-02-05T08:21:17Z

This is not the finished version for review. Before I finish, I'd like to know that this feature is required by bcc.
Can you please let me know if post unwind using dwarf unwind via libunwind is needed and it helps with issues like #3515?

FYI, there are a few things left to do for this PR if needed:

Added ARCH dependent code for x86
Dump user stack and regs when in kernel mode
Add libunwind for build
etc.

test environment
Tested with profile in aarch64 environment.
However, the profile's review PR is not in progress, so I apply it to offcputime to share the code.
This is the test results.
Before:

# ./profile -d -p `pidof test-strlen-abc-nofp-unwtbl`
Sampling at 49 Hertz of PID 83030 by user + kernel stack... Hit Ctrl-C to end.
^C    
    #0  0x0000007fbe74cc20 strerrordesc_np+0x1980 (/usr/lib/libc.so.6+0x9cc20)
    #1  0x0000007fbe6db30c __libc_start_main+0x9c (/usr/lib/libc.so.6+0x2b30c)
    #2  0x000000556f9c0754 _start+0x34 (/var/rootdirs/home/root/test-strlen-abc-nofp-unwtbl+0x754)
    -                test-strlen-abc (83030)
        290

After:

# ./profile -P -d -p `pidof test-strlen-abc-nofp-unwtbl`
Sampling at 49 Hertz of PID 83030 by user + kernel stack... Hit Ctrl-C to end.
^C    
    #0  0x0000007fbe74cc20 strerrordesc_np+0x1980 (/usr/lib/libc.so.6+0x9cc20)
    #1  0x000000556f9c087c c+0x30 (/var/rootdirs/home/root/test-strlen-abc-nofp-unwtbl+0x87c)
    #2  0x000000556f9c0840 b+0x8 (/var/rootdirs/home/root/test-strlen-abc-nofp-unwtbl+0x840)
    #3  0x000000556f9c082c a+0x8 (/var/rootdirs/home/root/test-strlen-abc-nofp-unwtbl+0x82c)
    #4  0x000000556f9c08c4 main+0x10 (/var/rootdirs/home/root/test-strlen-abc-nofp-unwtbl+0x8c4)
    #6  0x0000007fbe6db230 __libc_init_first+0x70 (/usr/lib/libc.so.6+0x2b230)
    -                test-strlen-abc (83030)
        1

test executable information
Test executable has no frame pointer, but has an unwind table (.eh_frame).
3.1 compile option : -fomit-frame-pointer -funwind-tables
3.2 code

#include <assert.h>
#include <stdlib.h>
#include <string.h>

char buf[8*1024*1024];

int a(void);
int b(void);
int c(void);

typedef size_t (*strlen_t)(const char *); 
strlen_t getlen = strlen;

int a(void)
{
        return b() - 1;
}

int b(void)
{
        return c() + 1;
}

int c() 
{
        memset(buf, '+', sizeof(buf)-1);

        while (1) {
                size_t len = getlen(buf);
                assert(len == sizeof(buf) - 1); 
        }

        // not reached
        return 1;
}

int main(int argc, char **argv)
{
        a();
}

Thank you.

yonghong-song · 2023-02-05T19:51:39Z

@davemarchevsky could you also take a look at this patch?

yonghong-song · 2023-02-13T05:35:08Z

I think this could be a useful feature. In most bcc tools, the user space gets stack_id and a list of addresses for the stack and then reconstruct the symbolized stack strace based recorded pid. If the application compiled with omitting frame pointer, it would be hard from the kernel to figure out the correct stack trace. In these cases, having a backup using user space libunwind seems a reasonable alternative. So please go ahead to finish your implementation. I see you use stack size 128. This may not be enough for real world big application which could be up to 4k. I think we should make stack size as part of the input for -P. If not specified, you could use 128 bytes.

ekyooo · 2023-02-13T08:52:32Z

Thank you for your answers and comments.
I'll go ahead to finish the implementation.
Thank you.

ekyooo · 2023-07-19T09:52:02Z

Not yet subject to review. Thank you.

ekyooo · 2023-09-06T11:07:51Z

libbpf-tools/Makefile

@@ -26,6 +29,10 @@ USE_BLAZESYM ?= 1
 endif
 endif

+ifeq ($(ARCH),x86)
+USE_LIBUNWIND = 1


Could you please tell me if LIBUNWIND should be enabled by default?

ekyooo · 2023-09-06T11:19:07Z

libbpf-tools/offcputime.c

+	case 'P':
+		env.unwind = true;
+		break;
+	case OPT_SAMPLE_STACK_SIZE:


I think we should make stack size as part of the input for -P. If not specified, you could use 128 bytes.

offcputime has a positional argument for the duration, so -P cannot be set to OPTION_ARG_OPTIONAL.
Therefore, a separate option for stack size has been added.

ekyooo · 2023-09-06T11:24:35Z

libbpf-tools/Makefile

@@ -26,6 +29,10 @@ USE_BLAZESYM ?= 1
 endif
 endif

+ifeq ($(ARCH),x86)


For now, only build libunwind for x86 is added.

This is a test example using the test application in the description.

# ./offcputime -P --sample-stack-size 1024 -p `pidof test-strlen-abc-nofp-unwtbl` ^C bpf_prog_99b48c45746a8a74_sched_switch bpf_prog_99b48c45746a8a74_sched_switch bpf_trace_run3 __traceiter_sched_switch __schedule schedule exit_to_user_mode_prepare irqentry_exit_to_user_mode asm_sysvec_apic_timer_interrupt strlen c b a main __libc_start_main _start [unknown] [unknown] - test-strlen-abc (26307) 50

test with -v option.

# ./offcputime -P -v --sample-stack-size 1024 -p `pidof test-strlen-abc-nofp-unwtbl` #0 0xffffffffc001259c bpf_prog_99b48c45746a8a74_sched_switch+0x4bc #1 0xffffffffc001259c bpf_prog_99b48c45746a8a74_sched_switch+0x4bc #2 0xffffffff8116952e bpf_trace_run3+0x3e #3 0xffffffff8109be54 __traceiter_sched_switch+0x34 #4 0xffffffff81c5f7ba __schedule+0x30a #5 0xffffffff81c5fbff schedule+0x3f #6 0xffffffff810f3766 exit_to_user_mode_prepare+0x56 #7 0xffffffff81c5d6d5 irqentry_exit_to_user_mode+0x5 #8 0xffffffff81e00c42 asm_sysvec_apic_timer_interrupt+0x12 #9 0x00007f9797d6fbc6 strlen+0x106 (/lib/x86_64-linux-gnu/libc-2.19.so+0x88bc6) #10 0x00005582b6600716 c+0x2a (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x716) #11 0x00005582b66006e4 b+0x9 (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x6e4) #12 0x00005582b66006d3 a+0x9 (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x6d3) #13 0x00005582b6600756 main+0x11 (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x756) #14 0x00007f9797d08ec5 __libc_start_main+0xf5 (/lib/x86_64-linux-gnu/libc-2.19.so+0x21ec5) #15 0x00005582b66005ea _start+0x2a (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x5ea) #16 0xffffffff81c5d6d5 (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x5ea) #17 0xffffffff81e00c42 (/root/es/unwind/test-strlen-abc-nofp-unwtbl+0x5ea) - test-strlen-abc (26307) 54

Thank you for waiting so long.
Could you please review this?
Thank you.

ekyooo · 2023-09-08T09:07:02Z

In the last update, this feature is only built if USE_LIBUNWIND is enabled in the Makefile.
Thank you.

Bojun-Seo · 2023-10-31T07:58:24Z

libbpf-tools/unwind_helpers.h

+#include <libunwind-ptrace.h>
+#include "unwind_types.h"
+
+enum log_level {


Is this a log level only for this feature(post unwinding)? Could you please tell me the role of this enum log_level?

Bojun-Seo · 2023-10-31T07:59:24Z

libbpf-tools/unwind_helpers.h

+	struct unw_data data;
+};
+
+int unw_init(struct unw_info *u, pid_t pid, size_t user_stack_size);


How about mentioning the meaning of each parameters for each APIs?

Bojun-Seo · 2023-10-31T08:03:19Z

libbpf-tools/offcputime.c

@@ -355,9 +427,25 @@ int main(int argc, char **argv)
 	 */
 	sleep(env.duration);

+#ifdef USE_LIBUNWIND


How about removing USE_LIBUNWIND macros?

Bojun-Seo · 2023-10-31T08:09:18Z

libbpf-tools/offcputime.c

+#ifdef USE_LIBUNWIND
+		if (env.unwind) {
+			if (post_unwind(u, sample_fd, ustack_fd, next_key.user_stack_id,
+					ip, env.perf_max_stack_depth) != 0) {


It would be nice if ip were the last parameter.

ekyooo · 2024-01-11T06:42:29Z

@Bojun-Seo
Thank you for your comment.
Including your feedback, I've found many things I need to fix and improve in this patch.
Well, I'll come back soon with a revised patch.
Thanks again.

…ng libunwind unwind.bpf.h: bpf helper API for user stack and register dumps unwind_helpers: helper API for post dwarf unwind using libunwind

Use installed libunwind (Only builds for x86 are supported in this patch.): check Signed-off-by: Eunseon Lee <[email protected]>

…wind_helpers Add -P option to use post dwarf unwind with libunwind. With the option, bpf program dump user stack and user regs. and user program get user stack traces using unwind_helpers to show output.

ekyooo requested review from drzaeus77, goldshtn, yonghong-song, 4ast, brendangregg and davemarchevsky as code owners February 5, 2023 08:21

ekyooo changed the title ~~libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind~~ [draft] libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind Feb 5, 2023

davemarchevsky self-assigned this Feb 9, 2023

ekyooo force-pushed the unwind_offcputime branch from c6e4ca1 to 74ad561 Compare July 19, 2023 06:44

ekyooo force-pushed the unwind_offcputime branch 2 times, most recently from 032adab to d689460 Compare September 6, 2023 11:03

ekyooo changed the title ~~[draft] libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind~~ libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind Sep 6, 2023

ekyooo commented Sep 6, 2023

View reviewed changes

ekyooo force-pushed the unwind_offcputime branch from d689460 to 8c28a91 Compare September 8, 2023 08:56

ekyooo mentioned this pull request Sep 28, 2023

libbpf-tools/offcputime: Add -s option to show similar user stack trace for processes without unwind table or fp. #4679

Open

Bojun-Seo reviewed Oct 31, 2023

View reviewed changes

This was referenced Dec 28, 2023

cannot see pthread_cond_wait on offcpu graph #4853

Open

why trace.py can not give full userstack with kernel stack #4735

Open

ekyooo force-pushed the unwind_offcputime branch from 8c28a91 to e1485ea Compare January 31, 2024 06:26

ekyooo marked this pull request as draft January 31, 2024 06:27

ekyooo added 3 commits April 17, 2024 16:03

libbpf-tools/unwind_helpers: Add helper API for post dwarf unwind usi…

2a25b68

…ng libunwind unwind.bpf.h: bpf helper API for user stack and register dumps unwind_helpers: helper API for post dwarf unwind using libunwind

libbpf-tools: Apply libunwind to offcputime

0e1d125

Use installed libunwind (Only builds for x86 are supported in this patch.): check Signed-off-by: Eunseon Lee <[email protected]>

libbpf-tools/offcputime: Add -P option for post dwarf unwind using un…

80cf8c0

…wind_helpers Add -P option to use post dwarf unwind with libunwind. With the option, bpf program dump user stack and user regs. and user program get user stack traces using unwind_helpers to show output.

ekyooo force-pushed the unwind_offcputime branch from e1485ea to 80cf8c0 Compare April 17, 2024 07:08

Bojun-Seo mentioned this pull request Apr 18, 2024

memleak: the symbol of the function that calls new disappeared #4958

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libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind #4463

libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind #4463

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ekyooo commented Jan 11, 2024

libbpf-tools/offcputime: Add -P option to post unwind user stack backtrace using dwarf unwind via libunwind #4463

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