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Date: Fri, 28 Jun 2024 20:27:55 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: duchangbin <changbin.du@...wei.com>, Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Nathan Chancellor <nathan@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
"Liang, Kan" <kan.liang@...ux.intel.com>,
Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
<morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [PATCH v4 1/5] perf: support specify vdso path in cmdline
On 28/06/24 07:21, duchangbin wrote:
> On Thu, Jun 27, 2024 at 04:53:18PM -0700, Namhyung Kim wrote:
>> Hello guys,
>>
>> On Wed, Jun 26, 2024 at 01:32:42PM +0300, Adrian Hunter wrote:
>>> On 26/06/24 05:26, duchangbin wrote:
>>>> On Tue, Jun 25, 2024 at 04:20:49PM +0300, Adrian Hunter wrote:
>>>>> On 25/06/24 06:37, Changbin Du wrote:
>>>>>> The vdso dumped from process memory (in buildid-cache) lacks debugging
>>>>>> info. To annotate vdso symbols with source lines we need specify a
>>>>>> debugging version.
>>>>>>
>>>>>> For x86, we can find them from your local build as
>>>>>> arch/x86/entry/vdso/vdso{32,64}.so.dbg. Or they may reside in
>>>>>> /lib/modules/<version>/vdso/vdso{32,64}.so on Ubuntu. But notice that
>>>>>> the buildid has to match.
>>>>>>
>>>>>> $ sudo perf record -a
>>>>>> $ sudo perf report --objdump=llvm-objdump \
>>>>>> --vdso arch/x86/entry/vdso/vdso64.so.dbg,arch/x86/entry/vdso/vdso32.so.dbg
>>>>>>
>>>>>> Samples: 17K of event 'cycles:P', 4000 Hz, Event count (approx.): 1760
>>>>>> __vdso_clock_gettime /work/linux-host/arch/x86/entry/vdso/vdso64.so.d
>>>>>> Percent│ movq -48(%rbp),%rsi
>>>>>> │ testq %rax,%rax
>>>>>> │ ; return vread_hvclock();
>>>>>> │ movq %rax,%rdx
>>>>>> │ ; if (unlikely(!vdso_cycles_ok(cycles)))
>>>>>> │ ↑ js eb
>>>>>> │ ↑ jmp 74
>>>>>> │ ; ts->tv_sec = vdso_ts->sec;
>>>>>> 0.02 │147: leaq 2(%rbx),%rax
>>>>>> │ shlq $4, %rax
>>>>>> │ addq %r10,%rax
>>>>>> │ ; while ((seq = READ_ONCE(vd->seq)) & 1) {
>>>>>> 9.38 │152: movl (%r10),%ecx
>>>>>>
>>>>>> When doing cross platform analysis, we also need specify the vdso path if
>>>>>> we are interested in its symbols.
>>>>>
>>>>> Would it be possible to add vdso and vdso debug to the build-id
>>>>> cache and ensure perf can find it there?
>>>>>
>>>>> Typically, getting dsos from another machine is handled via
>>>>> build-id cache e.g. what perf-archive does
>>>>>
>>>> Hmm. I agree this is better alternative approach for cross-machine analysis.
>>>> When collecting vdsos to buildid cache, I think we can use the local searched
>>>> objects (with debug symbols) instead if its build-id matches vdsos from process
>>>> dumping (the real code ran).
>>>>
>>>> Currently I just follow what perf does for vmlinux so to reuse most of existing
>>>> code. Maybe vmlinux is too big to add to buildid-cahce?
>>>>
>>>> Can we keep our current strategy for now? I'll think about above options when
>>>> I have more time.
>>>>
>>>
>>> I tried adding vdso via perf buildid-cache. It doesn't work only
>>> because the lookup expects the basename to be "vdso" but it is
>>> "elf".
>>>
>>> Adding a link from "vdso" to "elf" made it work e.g.
>>>
>>> $ cat gettimeofday-test.c
>>> #include <stdio.h>
>>> #include <sys/time.h>
>>>
>>> int main()
>>> {
>>> struct timeval tv;
>>> int ret;
>>>
>>> ret = gettimeofday(&tv, NULL);
>>> if (ret == -1) {
>>> fprintf(stderr, "gettimeofday failed\n");
>>> return 1;
>>> }
>>>
>>> printf("%lu.%lu\n", (unsigned long)tv.tv_sec, (unsigned long)tv.tv_usec);
>>>
>>> return 0;
>>> $ perf record -e intel_pt//u ./gettimeofday-test
>>> 1719397042.892837
>>> [ perf record: Woken up 1 times to write data ]
>>> [ perf record: Captured and wrote 0.026 MB perf.data ]
>>> $ perf script --itrace=e
>>> $ perf buildid-cache --remove /lib/modules/6.5.0-41-generic/vdso/vdso64.so
>>> $ perf script --itrace=e
>>> Warning:
>>> 2 instruction trace errors
>>> instruction trace error type 1 time 525345.386424204 cpu 4 pid 198976 tid 198976 ip 0x7ffddb0e8e00 code 5: Failed to get instruction
>>> instruction trace error type 1 time 525345.386424829 cpu 4 pid 198976 tid 198976 ip 0x7ffddb0e884d code 5: Failed to get instruction
>>> $ perf buildid-cache --add /lib/modules/6.5.0-41-generic/vdso/vdso64.so
>>> $ perf script --itrace=e
>>> Warning:
>>> 2 instruction trace errors
>>> instruction trace error type 1 time 525345.386424204 cpu 4 pid 198976 tid 198976 ip 0x7ffddb0e8e00 code 5: Failed to get instruction
>>> instruction trace error type 1 time 525345.386424829 cpu 4 pid 198976 tid 198976 ip 0x7ffddb0e884d code 5: Failed to get instruction
>>> $ cd ~/.debug/.build-id/c3/530aed66e71bfd10af66039f58cc7c4d2eaba8
>>> ~/.debug/.build-id/c3/530aed66e71bfd10af66039f58cc7c4d2eaba8$ ln -s elf vdso
>>> ~/.debug/.build-id/c3/530aed66e71bfd10af66039f58cc7c4d2eaba8$ ls -l
>>> total 36
>>> -rw-r--r-- 1 ahunter ahunter 33272 Jun 26 13:17 elf
>>> -rw-r----- 1 ahunter ahunter 0 Jun 26 13:17 probes
>>> lrwxrwxrwx 1 ahunter ahunter 3 Jun 26 13:18 vdso -> elf
>>> /.debug/.build-id/c3/530aed66e71bfd10af66039f58cc7c4d2eaba8$ cd
>>> $ perf script --itrace=e
>>> $
>>>
>>> So maybe a change could be made to build_id_cache__add() to add
>>> the extra link if the file name matches vdso
>>
>> Thanks for doing this! I noticed buildid_cache__basename() will handle
>> the name properly once it realizes the file is a vdso. Maybe we can
>> check the filepath with some patterns, or simply add an command line
>> option to say it's a vdso.
>>
>> Thanks,
>> Namhyung
>>
> I added some tricks for vdso in build_id_cache__add(). It replace vdso object
> with debugging one if found. Is this okay?
perf has support for storing debug files in the build-id
cache using the base name "debug" instead of "elf", so really
it would be better to make that work
>
> +static char *build_id_cache__find_debug_vdso(const char *sbuild_id)
> +{
> + char sbuild_id_tmp[SBUILD_ID_SIZE];
> + struct build_id bid;
> + int i, ret = 0;
> +
> + printf("Looking at the vdso_path (%d entries long)\n",
> + vdso_paths.nr_entries + 1);
> +
> + for (i = 0; i < vdso_paths.nr_entries; ++i) {
> + ret = filename__read_build_id(vdso_paths.paths[i], &bid);
> + if (ret < 0)
> + continue;
> +
> + build_id__sprintf(&bid, sbuild_id_tmp);
> + if (!strcmp(sbuild_id, sbuild_id_tmp)) {
> + printf("Found debugging vdso %s\n", vdso_paths.paths[i]);
> + return strdup(vdso_paths.paths[i]);
> + }
> + }
> +
> + return NULL;
> +}
> +
> int
> build_id_cache__add(const char *sbuild_id, const char *name, const char *realname,
> struct nsinfo *nsi, bool is_kallsyms, bool is_vdso,
> const char *proper_name, const char *root_dir)
> {
> const size_t size = PATH_MAX;
> - char *filename = NULL, *dir_name = NULL, *linkname = zalloc(size), *tmp;
> + char *filename = NULL, *dir_name = NULL, *vdso_name = NULL, *linkname = zalloc(size), *tmp;
> char *debugfile = NULL;
> int err = -1;
>
> + /* replace vdso object with debugging one if found */
> + if (is_vdso) {
> + vdso_name = build_id_cache__find_debug_vdso(sbuild_id);
> + if (vdso_name)
> + name = realname = vdso_name;
> + }
> +
> if (!proper_name)
> proper_name = name;
>
>>
>
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