Why doesn't "xargs sudo perf top" work as expected? - linux

Question

I want to profile a process, so first get its pid, then use "perf top" to check it:
$ pgrep program
14472
$ sudo perf top -p 14472
It works as expected:
Then I want to use pipe to connect these two commands, so I use xargs:
$ pgrep program | sudo xargs perf top -p
But this time "perf top" seems not work normally:
I compared processes of these two operations:
(1) Run pgrep and perf separately:
$ ps -ef | grep perf
root 18468 16827 0 09:34 pts/3 00:00:00 sudo perf top -p 14472
root 18469 18468 91 09:34 pts/3 00:00:06 perf top -p 14472
nanxiao 18477 18295 0 09:34 pts/4 00:00:00 grep --color=auto perf
(2) Use xargs to connect pgrep and perf:
$ ps -ef | grep perf
nanxiao 18250 16827 0 09:32 pts/3 00:00:00 xargs sudo perf top -p
root 18251 18250 0 09:32 pts/3 00:00:00 sudo perf top -p 14472
root 18252 18251 87 09:32 pts/3 00:01:47 perf top -p 14472
nanxiao 18442 18295 0 09:34 pts/4 00:00:00 grep --color=auto perf
IMHO, it seems same. Anyone can give some clues? Thanks in advance!
P.S., my OS is CentOS 7.

Answer 1

After checking manual again, I find -o option can fix this issue:
-o, --open-tty
Reopen stdin as /dev/tty in the child process before executing
the command. This is useful if you want xargs to run an
interactive application.
The command is like this:
$ pgrep program | sudo xargs -o perf top -p
But unfortunately, CentOS 7's xargs is a little old, and doesn't provide this option.
The root cause is: without -o option, the stdin of perf program is /dev/null:
$ sudo lsof -p 1495
......
perf 1495 root 0r CHR 1,3 0t0 2052 /dev/null
......
And the perf is blocked in SLang_getkey():
......
FD_ZERO(&read_set);
FD_SET(0, &read_set);
if (delay_secs) {
timeout.tv_sec = delay_secs;
timeout.tv_usec = 0;
}
err = select(1, &read_set, NULL, NULL, ptimeout);
if (err == 0)
return K_TIMER;
if (err == -1) {
if (errno == EINTR)
return K_RESIZE;
return K_ERROR;
}
key = SLang_getkey();
if (key != K_ESC)
return key;
......
Read of /dev/null will return EOF, then select() will return 1.
With -o option the stdin of perf program is /dev/tty:
$ sudo lsof -p 1394
......
perf 1394 root 0u CHR 136,25 0t0 28 /dev/pts/25
......
In above code, the select() will return 0, and the whole function will return accordingly.

Answer 2

A better approach would be to directly run the top on the output of pgrep instead of piping over xargs. I believe top command by default does not read information over standard input
sudo perf top -p "$(pgrep program)"
This way the $(..) returns the output of the pgrep command and the returned value is passed as a positional argument value to the -p flag.