Re: [PATCH v4 1/5] perf: support specify vdso path in cmdline

[duchangbin <changbin.du@huawei.com>]

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?

+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;

> 

-- 
Cheers,
Changbin Du