Why doesn't “xargs sudo perf top” work as expected?

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!

PS, 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.