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

[duchangbin]

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.

-- 
Cheers,
Changbin Du