perf symbols: Ignore stripped vmlinux and fallback to kallsyms

If a vmlinux is stripped, perf will use it and ignore kallsyms. We
end up with useless profiles where everything maps to a few
runtime symbols:

    63.39%       swapper  [kernel.kallsyms]   [k] hcall_real_table
     4.90%      beam.smp  [kernel.kallsyms]   [k] hcall_real_table
     4.44%      beam.smp  [kernel.kallsyms]   [k] __sched_text_start
     3.72%      beam.smp  [kernel.kallsyms]   [k] __run_at_kexec

Detect this case and fallback to using kallsyms. This fixes the issue:

    62.81%       swapper  [kernel.kallsyms]   [k] snooze_loop
     4.44%      beam.smp  [kernel.kallsyms]   [k] __schedule
     0.91%      beam.smp  [kernel.kallsyms]   [k] _switch
     0.73%      beam.smp  [kernel.kallsyms]   [k] put_prev_entity
Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20140909085929.4a5a81f0@krytenSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol-elf.c

@@ -717,6 +717,14 @@ int dso__load_sym(struct dso *dso, struct map *map,
 		symbols__delete(&dso->symbols[map->type]);
 
 	if (!syms_ss->symtab) {
+		/*
+		 * If the vmlinux is stripped, fail so we will fall back
+		 * to using kallsyms. The vmlinux runtime symbols aren't
+		 * of much use.
+		 */
+		if (dso->kernel)
+			goto out_elf_end;
+
 		syms_ss->symtab  = syms_ss->dynsym;
 		syms_ss->symshdr = syms_ss->dynshdr;
 	}