[CentOS] another bizarre thing...

Fri Aug 9 14:24:27 UTC 2019
Young, Gregory <gregory.young at solarwinds.com>

Hi Fred,

Yep, that's exactly how control groups work in CentOS 7. You don't need to define them (normally), they get assigned when the init script or systemd service launches it. As I mentioned, the idea is to ensure none of those child threads become zombies if the parent dies/crashes/gets killed. For troubleshooting, you could try moving the child threads into their own cgroup, which might help reduce the noise when the parent process gets killed. Of course, you will have to manually kill the child processes during this testing, but it might clear enough of the strace logging for you to see where the parent process is getting killed. Don't forget to undo this debugging step when done, or you will end up with zombies when you legitimately want to shut down the process.

Also, if you haven't already, you may want to convert it to use the systemd ".service" file launching. It gives you a lot of control over startup timeouts, restarts, shutdown commands, process branching, etc. if nothing else, it might help you identify when the process dies, and restart it without intervention...

Gregory Young 

-----Original Message-----
From: CentOS <centos-bounces at centos.org> On Behalf Of Fred Smith
Sent: August 8, 2019 7:48 PM
To: centos at centos.org
Subject: Re: [CentOS] another bizarre thing...

On Thu, Aug 08, 2019 at 05:06:06PM +0000, Young, Gregory wrote:
> Is this on both EL6 and EL7? If only EL7, it could be control groups causing the issue. The idea of cgroups is to prevent zombie processes, but if you need your program to spawn another process then restart itself while the other process continues to run, you need to launch it in a different control group, or the shutdown of the parent process will also kill the child. In my case, we have an upgrade script which needs to get called, then shut down the calling process in order to upgrade it. For example:
> # Clear any errors in the upgrade control group.
> /bin/systemctl reset-failed upgrade-trigger)
> # Launch the upgrader in its own control group.
> /bin/systemd-run --unit=upgrade-trigger --slice=upgrade-trigger /bin/bash /opt/myapp/Upgrade.sh "$1" "$2"
> If we don't do this, the upgrade fails as the upgrader get's terminated when the parent application is shut down.

well, we aren't INTENTINALLY using control groups. do we get put into one by the very act of launching a program w hich then creates threads, and they then all coexist until they're told to stop?

I think it's not the scenario you describe, the main program launches from an init script, does some sanity checks, loads some config files, then spawns the number of threads defined by its configuration. then all the threads, including the main prog, hang around doing stuff until they're told to stop, which happens all at once for all of them.
On a good day, anyway. what is happening now is they will all run fine for some time (anhour or twelve) then they all receive a SIGKILL.

Accordiing to a systemtap script I found online, it thinks the program is killing itself, but as the guy who wrote it, I don't think so.
the script can be seen below in earlier mail.

As for if it also fails on C6, I don't know. I've asked our support team to see if they have a C6/EL6 customer who will let them install the latest version for 6 and see what happens, but so far, no joy.


> Subject: Re: [CentOS] another bizarre thing...
> On Mon, Aug 05, 2019 at 08:57:45PM -0400, Fred Smith wrote:
> > Hi all!
> > 
> > I'm stuck on something really bizarre that is happening to a product 
> > I "own" at work. It's a C program, built on CentOS, runs on CentOs 
> > or RHEL, has been in circulation since the early 00's, is in use at 
> > hundreds of sites.
> > 
> > recently, at multiple customer sites it has started just going away.
> > no core file (yes, ulimit is configured), nothing in any of its
> > (several) log files. it's just gone.
> > 
> > running it under strace until it dies reveals that every thread has 
> > been given a SIGKILL.
> > 
> > How does one figure out who deliverd a SIGKILL? For other, 
> > non-fatal, signals it is possible to glean the PID of the sending 
> > process in a signal  handler, but obviously you can't do that for 
> > SIGKILL because the app doesn't survive the signal.
> > 
> > I'm grasping at straws here, and am open to almost any kind of 
> > suggestion that can be followed-up (as compared to "beats me" which 
> > is where I am now).
> OK, more information.
> Found a recipe to cause systemtap to emit a line of text identifying the sender of the SIGKILL.
> probe signal.send {
>   if (sig_name == "SIGKILL")
>     printf("%s was sent to %s (pid:%d) by %s uid:%d\n",
>            sig_name, pid_name, sig_pid, execname(), uid())
> unfortunately, it says the program is killing itself:
> 	SIGKILL was sent to myprog (pid:12269) by myprog uid:1000
> So,... now I'm wondering how one figures that out. nowhere in my source code does it explicitly raise any signal, much less SIGKILL.
> So there must be some underlying library or system call or something doing it.
> --
> ---- Fred Smith -- fredex at fcshome.stoneham.ma.us -----------------------------
>                        I can do all things through Christ 
>                               who strengthens me.
> ------------------------------ Philippians 4:13 
> ------------------------------- 
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---- Fred Smith -- fredex at fcshome.stoneham.ma.us -----------------------------
  "And he will be called Wonderful Counselor, Mighty God, Everlasting Father,
  Prince of Peace. Of the increase of his government there will be no end. He  will reign on David's throne and over his kingdom, establishing and upholding
      it with justice and righteousness from that time on and forever."
------------------------------- Isaiah 9:7 (niv) ------------------------------ _______________________________________________
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