[PATCH] perf stat: wait for the correct child

[Milian Wolff]

When packaging the perf userland application into an AppImage,
the wait() call in perf stat returned too early. It turned out
that some other child process exited, but not the one perf stat
launched:

~~~~~
$ sudo strace -e fork,execve,clone,wait4 -f ./perf-x86_64.AppImage stat sleep 1
execve("./perf-git.3a73b7f9-x86_64.AppImage", ["./perf-git.3a73b7f9-x86_64.AppIm"..., "stat", "sleep", "1"], 0x7ffec1bbf050 /* 18 vars */) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f6a6e7efe50) = 3912
strace: Process 3912 attached
[pid  3912] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f6a6e7efe50) = 3914
strace: Process 3914 attached
[pid  3912] +++ exited with 0 +++
[pid  3911] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=3912, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
[pid  3914] clone(strace: Process 3915 attached
child_stack=0x7f6a6d9fefb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f6a6d9ff9d0, tls=0x7f6a6d9ff700, child_tidptr=0x7f6a6d9ff9d0) = 3915
[pid  3911] execve("/tmp/.mount_perf-g6VYMpl/AppRun", ["./perf-git.3a73b7f9-x86_64.AppIm"..., "stat", "sleep", "1"], 0x14aab70 /* 21 vars */) = 0
[pid  3911] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f4ae113c4d0) = 3916
strace: Process 3916 attached
[pid  3911] wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 3912
[pid  3916] execve("/usr/libexec/perf-core/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/tmp/./sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/home/milian/.bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/usr/lib/icecream/libexec/icecc/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/ssd2/milian/projects/compiled/other/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/home/milian/.bin/kf5/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/ssd2/milian/projects/compiled/kf5/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/home/milian/projects/compiled/other/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/home/milian/projects/compiled/kf5/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/usr/local/sbin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/usr/local/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */) = -1 ENOENT (No such file or directory)
[pid  3916] execve("/usr/bin/sleep", ["sleep", "1"], 0x27d3650 /* 22 vars */
 Performance counter stats for 'sleep 1':

     <not counted>      task-clock
     <not counted> ) = 0
     context-switches
     <not counted>      cpu-migrations
     <not counted>      page-faults
     <not counted>      cycles
     <not counted>      instructions
     <not counted>      branches
     <not counted>      branch-misses

       0.000047194 seconds time elapsed

[pid  3916] --- SIGTERM {si_signo=SIGTERM, si_code=SI_USER, si_pid=3911, si_uid=0} ---
[pid  3916] +++ killed by SIGTERM +++
[pid  3911] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=3916, si_uid=0, si_status=SIGTERM, si_utime=0, si_stime=0} ---
[pid  3915] --- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=3914, si_uid=0} ---
[pid  3911] +++ exited with 0 +++
[pid  3915] --- SIGHUP {si_signo=SIGHUP, si_code=SI_USER, si_pid=3914, si_uid=0} ---
[pid  3915] +++ exited with 0 +++
+++ exited with 0 +++
~~~~~

This patch uses waitpid instead to ensure the call waits for the
debuggee application launched by perf stat. This fixes perf stat
when launched from an AppImage:

~~~~~
$ ./perf-x86_64.AppImage stat sleep 1

 Performance counter stats for 'sleep 1':

          0.357235      task-clock (msec)         #    0.000 CPUs utilized
                 1      context-switches          #    0.003 M/sec
                 0      cpu-migrations            #    0.000 K/sec
                50      page-faults               #    0.140 M/sec
           1269602      cycles                    #    3.554 GHz
            654278      instructions              #    0.52  insn per cycle
            129963      branches                  #  363.803 M/sec
              7082      branch-misses             #    5.45% of all branches

       1.000633420 seconds time elapsed
~~~~~

Signed-off-by: Milian Wolff <milian.wolff@xxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: 4549240.M84I420bcG@milian-kdab2">http://lkml.kernel.org/r/4549240.M84I420bcG@milian-kdab2
---
 tools/perf/builtin-stat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 2248ebe582ca..88f1d5fbdb48 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -752,7 +752,7 @@ static int __run_perf_stat(int argc, const char **argv)
 				process_interval();
 			}
 		}
-		wait(&status);
+		waitpid(child_pid, &status, 0);
 
 		if (workload_exec_errno) {
 			const char *emsg = str_error_r(workload_exec_errno, msg, sizeof(msg));
-- 
2.14.1