[Bug 1561658] Re: “Cannot fork” and "Resource temporary unavailable"

[Gustavo Romero]

Using this Makefile with "make -j500" will trigger the problematic
behavior described in this bug.

** Attachment added: "Makefile to reproduce the issue described"
   https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1561658/+attachment/4610125/+files/Makefile

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https://bugs.launchpad.net/bugs/1561658

Title:
  “Cannot fork” and "Resource temporary unavailable"

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  On Ubuntu 16.04/ppc64el, the cgroup for a user session (bash) inherits
  from a sshd.service, when the user logs into the machine using SSH.

  This causes the amount of process to be limited by
  /etc/systemd/system/conf DefaultTasksMax=512

  This does not seem to happen on amd64. This is a cgroup tree diff:

  On x64, bash (in this case, PID 19405 ) spawned by sshd belongs to CGROUP session-5.scope->user-1003.slice->user.slice: 
   
  └─user.slice
    ├─user-1000.slice
    │ ├─session-1.scope
    │ │ ├─634 sshd: brenohl [priv]
    │ │ ├─660 sshd: brenohl at pts/0 
    │ │ └─661 -bash
    │ └─user at 1000.service
    │   ├─636 /lib/systemd/systemd --user
    │   └─637 (sd-pam)  
    └─user-1003.slice
      ├─session-5.scope
      │ ├─19379 sshd: gromero [priv]
      │ ├─19404 sshd: gromero at pts/1 
      │ ├─19405 -bash

  However, in ppc64le, bash (in this case, PID 1913), spawned by sshd
  belongs to CGROUP ssh.service->system.slice->-.slice:

  -.slice
  ├─1720 /sbin/cgmanager -m name=systemd
  ├─init.scope
  └─system.slice
    ├─dbus.service
    │ └─1699 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation
    ├─cron.service
    │ └─1702 /usr/sbin/cron -f
    ├─ifup at eth0.service
    │ └─1833 /sbin/dhclient 
    ├─accounts-daemon.service
    │ └─1717 /usr/lib/accountsservice/accounts-daemon
    ├─system-serial\x2dgetty.slice
    │ └─serial-getty at hvc0.service
    │   └─1875 /sbin/agetty --keep-baud 115200 38400 9600 hvc0 vt220
    ├─systemd-journald.service
    │ └─1382 /lib/systemd/systemd-journald
    ├─systemd-timesyncd.service
    │ └─1639 /lib/systemd/systemd-timesyncd
    ├─ssh.service
    │ ├─1863 /usr/sbin/sshd -D
    │ ├─1897 sshd: gromero [priv]
    │ ├─1912 sshd: gromero at pts/0 
    │ ├─1913 -bash

  
  Having the user session associated with the systemd cgroups (/system.slice/ssh.service) instead of normal user/session cgroups (as user-XXXX.slice/session-5.scope), causes the process to be limited to the systemd TasksMax limit, thus, causing "Cannot fork" and "Resource temporary unavailable" problems when the amount of processes reaches this 512 limit.

  Gustavo Romero has more details about this problem, and will comment
  soon.

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