[Bug 1904369] Re: systemd 100% CPU; constantly reading service for socket activation

[Haw Loeung]

Sorry about not providing further details on this bug. We ended up
switching the last of our systems to xinetd so can no longer reproduce
and provide the information required.

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https://bugs.launchpad.net/bugs/1904369

Title:
  systemd 100% CPU; constantly reading service for socket activation

Status in systemd package in Ubuntu:
  Incomplete
Status in systemd source package in Bionic:
  Incomplete

Bug description:
  Hi,

  We recently switched to using systemd's socket activation for per-
  client limits. The configs are as follows:

  | $ ls -la /etc/systemd/system/sockets.target.wants/rsyncd.socket
  | lrwxrwxrwx 1 root root 33 Aug 25 00:09 /etc/systemd/system/sockets.target.wants/rsyncd.socket -> /etc/systemd/system/rsyncd.socket

  | $ cat /etc/systemd/system/sockets.target.wants/rsyncd.socket
  | [Unit]
  | Description=rsync daemon (socket)
  | Conflicts=rsyncd.service
  |
  | [Socket]
  | ListenStream=873
  | Accept=yes
  | MaxConnections=85
  | MaxConnectionsPerSource=5
  | KeepAlive=true
  |
  | [Install]
  | WantedBy=sockets.target

  | $ cat /etc/systemd/system/rsyncd at .service
  | [Unit]
  | Description=rsync daemon
  | ConditionPathExists=/etc/rsyncd.conf
  |
  | [Service]
  | ExecStart=/usr/bin/rsync --daemon
  | StandardInput=socket
  |
  | [Install]
  | WantedBy=multi-user.target

  After a while, systemd runs hot consuming a whole CPU core:

  | $ top -p 1
  |   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
  |     1 root      20   0 2057476 251560   6264 R  99.7  0.1  17903:32 systemd

  strace shows it constantly doing this:

  | $ sudo strace -p 1
  | ...
  | openat(AT_FDCWD, "/etc/systemd/system/rsyncd at .service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = 14
  | fcntl(14, F_GETFL)                      = 0x28000 (flags O_RDONLY|O_LARGEFILE|O_NOFOLLOW)
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | read(14, "[Unit]\nDescription=rsync daemon\n"..., 4096) = 173
  | read(14, "", 4096)                      = 0
  | close(14)                               = 0
  | openat(AT_FDCWD, "/etc/systemd/system/rsyncd at .service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = 14
  | fcntl(14, F_GETFL)                      = 0x28000 (flags O_RDONLY|O_LARGEFILE|O_NOFOLLOW)
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | fstat(14, {st_mode=S_IFREG|0644, st_size=173, ...}) = 0
  | read(14, "[Unit]\nDescription=rsync daemon\n"..., 4096) = 173
  | read(14, "", 4096)                      = 0
  | close(14)                               = 0
  | ^Cstrace: Process 1 detached

  This is with 237-3ubuntu10.43 on a host running Bionic.

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