systemd ignores drop-in configuration files - what am I doing wrong?

On one of my machines with Debian Buster (which ships **systemd 241**), I wanted to watch resource usage via systemd-cgtop. When I started this utility, I was seeing memory usage, but neither CPU usage nor I/O usage. Obviously, CPU accounting was turned off. Following the manpage for system.conf , I put the this line into /etc/systemd/system.conf (all other lines already were commented out): DefaultCPUAccounting=yes This worked as expected (of course after having reloaded systemd itself by systemctl daemon-reexec). [ Note: In fact, I was still seeing CPU usage only for some slices, not for all, but this is another story / question. ] However, that man page does not recommend to change /etc/systemd/system.conf. Rather, we should create a drop-in configuration file with the required lines. I followed that advice, created the directory /etc/systemd/system.conf.d, and created a file /etc/systemd/system.conf.d/10-pp.conf. Then I removed the line shown above from /etc/systemd/system.conf, put it into /etc/systemd/system.conf.d/10-pp.conf, and issued systemctl daemon-reexec. This took me back to the beginning: systemd-cgtop didn't show CPU usage at all. I can reproduce the situation at any time. Regardless of the drop-in configuration file, I must alter the main configuration file to enable CPU accounting. What am I doing wrong? P.S. - I have verified that there is no other drop-in configuration file which could hurt mine. That is, /usr/lib/systemd/system.conf.d/ does not exist, nor does /usr/local/lib/systemd/system.conf.d/, nor does /run/systemd/system.conf.d/. - I have verified the access permissions of the directory and the file I have created. They are like the permissions of the other (installed-by-default) .d directories and the files in them, respectively.