Sorry for the late reply here. Got caught up in other stuff. I did some digging around. Set the loglevel with systemd-analyze to debug and also added the debug flag to pam_systemd.so.
What I noticed is that on boot, when 5000+ machines try to authenticate at once, pam_systemd seems to fall on its ass and fail due to resource exhaustion. An excerpt from journalctl is attached below. I thought this might've been the reason behind sshd processes not being assigned to the correct slice, but the processes for which these log entries are generated are not available on the system anymore, which I take as meaning that the sshd process exited because it couldn't open a session. I tried to go about it the other way around and search for logs generated by any sshd process which is under the system.slice. I used this oneliner to do so: for i in $(systemctl status ssh | grep 'sshd: <user>' | sed -E 's/[^0-9]*([0-9]*)[^0-9]*/\1/'); do echo "==> logs for process $i"; journalctl | grep '\[$i\]'; done | less The search came up empty. For none of the currently 7000+ sshd processes which aren't in the correct user slice, there are any logs in journalctl. I verified and all logs since boot time are currently still in the journal, it hasn't rotated yet. Jun 04 09:50:17 <hostname> sshd[8415]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8691]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8717]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28538]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8652]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8721]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8716]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28121]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[28551]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8682]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8396]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28524]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8719]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28546]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8690]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8697]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[7350]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8522]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8406]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28636]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[28620]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[28669]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8693]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8723]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28633]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8401]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8692]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28670]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8543]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8234]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8654]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28545]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8791]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8840]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8815]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28710]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8541]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28705]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[28269]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable Jun 04 09:50:17 <hostname> sshd[8725]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8786]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8849]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8562]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8572]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8816]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8870]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[8560]: pam_systemd(sshd:session): Failed to create session: Connection timed out Jun 04 09:50:17 <hostname> sshd[28759]: pam_systemd(sshd:session): Failed to connect to system bus: Resource temporarily unavailable
