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

[Namhyung Kim <namhyung@kernel.org>]

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;
-- 
2.1.2