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

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

On Fri, Jun 28, 2024 at 08:27:55PM +0300, Adrian Hunter wrote:
> 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
>
My thoughts are:
  1. add debugging vdso file as "debug" in buildid cache.
  2. add a new perf config item 'annotate.prefer_debug_file' (default false) to
     annotate with debugging object files.

-- 
Cheers,
Changbin Du