tools build feature: Check if eventfd() is available

A new 'perf bench epoll' will use this, and to disable it for older
systems, add a feature test for this API.

This is just a simple program that if successfully compiled, means that
the feature is present, at least at the library level, in a build that
sets the output directory to /tmp/build/perf (using O=/tmp/build/perf),
we end up with:

$ ls -la /tmp/build/perf/feature/test-eventfd*
-rwxrwxr-x. 1 acme acme 8176 Nov 21 15:58 /tmp/build/perf/feature/test-eventfd.bin
-rw-rw-r--. 1 acme acme 588 Nov 21 15:58 /tmp/build/perf/feature/test-eventfd.d
-rw-rw-r--. 1 acme acme 0 Nov 21 15:58 /tmp/build/perf/feature/test-eventfd.make.output
$ ldd /tmp/build/perf/feature/test-eventfd.bin
linux-vdso.so.1 (0x00007fff3bf3f000)
libc.so.6 => /lib64/libc.so.6 (0x00007fa984061000)
/lib64/ld-linux-x86-64.so.2 (0x00007fa984417000)
$ grep eventfd -A 2 -B 2 /tmp/build/perf/FEATURE-DUMP
feature-dwarf=1
feature-dwarf_getlocations=1
feature-eventfd=1
feature-fortify-source=1
feature-sync-compare-and-swap=1
$

The main thing here is that in the end we'll have -DHAVE_EVENTFD in
CFLAGS, and then the 'perf bench' entry needing that API can be
selectively pruned.

Cc: Adrian Hunter <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Jason Baron <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/Makefile.feature

···
         backtrace                       \
         dwarf                           \
         dwarf_getlocations              \
+        eventfd                         \
         fortify-source                  \
         sync-compare-and-swap           \
         get_current_dir_name            \

tools/build/feature/Makefile

···
          test-bionic.bin                        \
          test-dwarf.bin                         \
          test-dwarf_getlocations.bin            \
+         test-eventfd.bin                       \
          test-fortify-source.bin                \
          test-sync-compare-and-swap.bin         \
          test-get_current_dir_name.bin          \
···
 
 $(OUTPUT)test-libelf.bin:
 	$(BUILD) -lelf
+
+$(OUTPUT)test-eventfd.bin:
+	$(BUILD)
 
 $(OUTPUT)test-get_current_dir_name.bin:
 	$(BUILD)

tools/build/feature/test-all.c

···
 # include "test-dwarf_getlocations.c"
 #undef main
 
+#define main main_test_eventfd
+# include "test-eventfd.c"
+#undef main
+
 #define main main_test_libelf_getphdrnum
 # include "test-libelf-getphdrnum.c"
 #undef main
···
 	main_test_glibc();
 	main_test_dwarf();
 	main_test_dwarf_getlocations();
+	main_test_eventfd();
 	main_test_libelf_getphdrnum();
 	main_test_libelf_gelf_getnote();
 	main_test_libelf_getshdrstrndx();

tools/build/feature/test-eventfd.c

···
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2018, Red Hat Inc, Arnaldo Carvalho de Melo <[e-mail]>
+
+#include <sys/eventfd.h>
+
+int main(void)
+{
+	return eventfd(0, EFD_NONBLOCK);
+}

tools/perf/Makefile.config

···
   endif
 endif
 
+ifeq ($(feature-eventfd), 1)
+  CFLAGS += -DHAVE_EVENTFD
+endif
+
 ifeq ($(feature-get_current_dir_name), 1)
   CFLAGS += -DHAVE_GET_CURRENT_DIR_NAME
 endif
-
 
 ifdef NO_LIBELF
   NO_DWARF := 1