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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 07, 2014 at 02:20:04PM +0900, Namhyung Kim wrote:
> 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:
> 
>   # 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;

Why does this live in the minimal implementation; should we not always
discard ELF files with 0 symbols?

Suppose I have a vmlinux but removed all symbols from it; I want it to
fall back to kallsyms too.