Re: [PATCH 1/3] perf tools: Allow vmlinux to fallback to kallsyms on NO_LIBELF=1

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Nov 07, 2014 at 02:20:04PM +0900, Namhyung Kim escreveu:
> When libelf is not used, perf cannot not show symbol names since it
> doesn't access the ELF symbol table.  But kernel is different as it
> can fallback to kallsyms.
> 
> It worked w/o libelf, but recent change to use vmlinux for kernel
> symbols break it.
> 
> With this change, it now can show kernel symbols again:

Ok, but since you added that minimal ELF symtab loading, isn't better to
try that first, i.e. if we find a vmlinux file with a build-id and with
symbols in it...

If that isn't the case, i.e. no vmlinux was found, then we fallback to
kallsyms, to check if that is available.

I.e. with your new minimalistic ELF symtab loader if we have a suitable
vmlinux but no kallsyms, we end up resolving no symbols even having that
nice vmlinux :-)

Ah, I applied the feature test one (2/3, I think)

- Arnaldo

>   # Overhead  Command  Shared Object      Symbol
>   # ........  .......  .................  ........................
>   #
>       34.51%  swapper  [kernel.kallsyms]  [k] intel_idle
>       12.54%  perf     [kernel.kallsyms]  [k] generic_exec_single
>       10.11%  swapper  [kernel.kallsyms]  [k] int_sqrt
>        9.83%  swapper  [kernel.kallsyms]  [k] hrtimer_interrupt
>        7.25%  emacs    [kernel.kallsyms]  [k] __switch_to
>        7.06%  sleep    [kernel.kallsyms]  [k] find_next_iomem_res
>        7.02%  swapper  [kernel.kallsyms]  [k] run_timer_softirq
>        2.55%  swapper  [kernel.kallsyms]  [k] _raw_spin_unlock_irq
>        1.90%  swapper  [kernel.kallsyms]  [k] lapic_next_deadline
>        1.75%  swapper  [kernel.kallsyms]  [k] native_sched_clock
>        1.49%  swapper  [kernel.kallsyms]  [k] __schedule
>        1.20%  swapper  [kernel.kallsyms]  [k] read_tsc
>        1.10%  sleep    [kernel.kallsyms]  [k] current_kernel_time
>        0.99%  swapper  [kernel.kallsyms]  [k] pick_next_task_rt
>        0.36%  swapper  [kernel.kallsyms]  [k] group_sched_in
>        0.24%  swapper  [kernel.kallsyms]  [k] x86_pmu_commit_txn
>        0.06%  swapper  [kernel.kallsyms]  [k] intel_pmu_enable_all
>        0.03%  perf     [kernel.kallsyms]  [k] intel_pmu_enable_all
> 
> Reported-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/symbol-minimal.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/perf/util/symbol-minimal.c b/tools/perf/util/symbol-minimal.c
> index c9541fea9514..226cf41ed7e6 100644
> --- a/tools/perf/util/symbol-minimal.c
> +++ b/tools/perf/util/symbol-minimal.c
> @@ -335,6 +335,9 @@ int dso__load_sym(struct dso *dso, struct map *map __maybe_unused,
>  	unsigned char *build_id[BUILD_ID_SIZE];
>  	int ret;
>  
> +	if (dso->kernel)
> +		return 0;  /* always use kallsyms */
> +
>  	ret = fd__is_64_bit(ss->fd);
>  	if (ret >= 0)
>  		dso->is_64_bit = ret;
> -- 
> 2.1.2