On 09/05/2014 07:59 AM, Namhyung Kim wrote:
> There's a problem on finding correct kernel symbols when perf report
> runs on a different kernel. Although a part of the problem was solved
> by the prior commit 0a7e6d1b6844 ("perf tools: Check recorded kernel
> version when finding vmlinux"), there's a remaining problem still.
>
> When perf records samples, it synthesizes the kernel map using
> machine__mmap_name() and ref_reloc_sym like "[kernel.kallsyms]_text".
> You can easily see it using 'perf report -D' command.
>
> After finishing record, it goes through the recorded events to find
> maps/dsos actually used. And then record build-id info of them.
>
> During this process, it needs to load symbols in a dso and it'd call
> dso__load_vmlinux() since the default value of the symbol_conf.try_
> vmlinux_path is true. However it changes dso->long_name to a real
> path of the vmlinux file (e.g. /lib/modules/3.16.0-rc2+/build/vmlinux)
> if one is running on a custom kernel.
>
> It resulted in that perf report reads the build-id of the vmlinux, but
> cannot use it since it only knows about the [kernel.kallsyms] map. It
> then falls back to possible vmlinux paths by using the recorded kernel
> version (in case of a recent version) or a running kernel silently.
>
> Even with the recent tools, this still has a possibility of breaking
> the result. As the build directory is a symbolic link, if one built a
> new kernel in the same directory with different source/config, the old
> link to vmlinux will point the new file. So it's absolutely needed to
> use build-id when finding a kernel image.
>
> In this patch, it's now changed to try to search a kernel dso using
> "vmlinux" shortname (which always has a build-id) and, if not found,
> search "[kernel.kallsyms]".
>
> Before:
>
> $ perf report
> # Children Self Command Shared Object Symbol
> # ........ ........ ....... ................. ...............................
> #
> 72.15% 0.00% swapper [kernel.kallsyms] [k] set_curr_task_rt
> 72.15% 0.00% swapper [kernel.kallsyms] [k] native_calibrate_tsc
> 72.15% 0.00% swapper [kernel.kallsyms] [k] tsc_refine_calibration_work
> 71.87% 71.87% swapper [kernel.kallsyms] [k] module_finalize
> ...
>
> After (for the same perf.data):
>
> 72.15% 0.00% swapper vmlinux [k] cpu_startup_entry
> 72.15% 0.00% swapper vmlinux [k] arch_cpu_idle
> 72.15% 0.00% swapper vmlinux [k] default_idle
> 71.87% 71.87% swapper vmlinux [k] native_safe_halt
> ...
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> tools/perf/util/machine.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index b2ec38bf211e..d9f828f3b54f 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -1060,10 +1060,14 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
> strlen(kmmap_prefix));
> /*
> * Should be there already, from the build-id table in
> - * the header.
> + * the header (but maybe with a different name: "vmlinux").
> */
> - struct dso *kernel = __dsos__findnew(&machine->kernel_dsos,
> - kmmap_prefix);
> + struct dso *kernel = dsos__find(&machine->kernel_dsos,
> + "vmlinux", true);
Isn't "vmlinux" just the basename of the original file name, so if it had a
different name this wouldn't work e.g. if the filename had been
/boot/vmlinuz-3.11.0-26-generic then you would need
"vmlinuz-3.11.0-26-generic" for this to work?
> +
> + if (kernel == NULL)
> + kernel = __dsos__findnew(&machine->kernel_dsos,
> + kmmap_prefix);
> if (kernel == NULL)
> goto out_problem;
>
>
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