Re: [PATCH] perf header: Set proper symbol name for vdso when build-id event found

[Like Xu]

On 14/12/2023 3:26 am, Adrian Hunter wrote:
> On 1/12/23 13:15, Like Xu wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > If using perf-record to sample a cpu-cycles:P event on a hypervisor process
> > when '--kcore' is not enabled, user may find some surprise in perf-report:
> >
> > # perf report -i perf.data -v:
> > # Overhead Command   Shared Object            	Symbol
> >      99.71%  vcpu0    arch/x86/kvm/kvm-intel.ko  0xffffffffa10d1e30 B [k] 0x0000000000034ed0
> >
> > build id event received for vmlinux: d12116149f511f7dbd0b21c45d38d3d2ec09b87f [20]
> > build id event received for kvm-intel.ko: a8fc0213abbafd97b10ce58ce84bec8519f9abce [20]
> > build id event received for [vdso]: 4d56e381df8d2c051f6bc1ef69c0118c59d5c49f [20]
> >
> > # perf report:
> > # Overhead  Command  Shared Object     Symbol
> > # ........  .......  ................  .......................................
> > #
> >      99.71%  vcpu0    [kvm_intel]       [k] 0x0000000000034ed0
> >       0.10%  vcpu0    [kernel.vmlinux]  [k] __lock_acquire.isra.29
> >
> > Users may be curious as to how 0x34ed0 was generated and wondered if this
> > RIP came from the guest application but perf-script-D does not point to any
> > samples of this address.
> >
> > Based on perf/tool implementation, this is actually an offset pointing to
> > the vdso object (in this case it is the assembly __vmx_vcpu_run defined
> > in arch/x86/kvm/vmx/vmenter.S). The pattern is not reproduced on perf-tool
> > of some distributions, and git-bisect quickly identified the possible root
> > cause, which leds to this straightforward fix and after this change:
> >
> > # perf report -i perf.data -v:
> > # Overhead Command   Shared Object            	Symbol
> >      99.71%  vcpu0    arch/x86/kvm/kvm-intel.ko  0x34ed0            B [k] __vmx_vcpu_run
> >
> > # perf report:
> > # Overhead  Command  Shared Object     Symbol
> > # ........  .......  ................  .......................................
> > #
> >      99.71%  vcpu0    [kvm_intel]       [k] __vmx_vcpu_run
> >
> > The fix also gets commit 1deec1bd96cc ("perf header: Set proper module name
> > when build-id event found") lit again.
> >
> > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Cc: Ian Rogers <irogers@xxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Fixes: b2fe96a350de ("perf tools: Fix module symbol processing")
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
> >   tools/perf/util/header.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index e86b9439ffee..a33d589511ff 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -2305,8 +2305,8 @@ static int __event_process_build_id(struct perf_record_header_build_id *bev,
> >   
> >   			if (!kmod_path__parse_name(&m, filename) && m.kmod)
> >   				dso__set_module_info(dso, &m, machine);
> > -
> > -			dso->kernel = dso_space;
> > +			else
> > +				dso->kernel = dso_space;
>
> This is undoing some of b2fe96a350de ("perf tools: Fix module
> symbol processing") without explanation.

Thanks for your comments.
W/ this fix, "perf test -v object" looks OK.

> Symbols in the .noinstr.text section don't seem to be
> being resolved, so that could be the issue.  perf synthesizes
> an MMAP record from /proc/modules, which works for .text
> but perhaps not for .noinstr.text

Not sure if it covers the potentially broken features you mentioned above,
would you be willing to provide a more detailed test command ?

> >   			free(m.name);
> >   		}
> >