The reason why it is fast under upstart is that with upstart the entire
session is running in the logind scope/session, in particular unity-
settings-daemon; and then pkexec causes

Okt 06 22:10:46 donald pkexec[9943]: pam_systemd(polkit-1:session):
Cannot create session: Already running in a session

This is why the entire pam_systemd, systemd --user etc. for root are
entirely skipped.

Under the "user session" paradigm that we get with dbus-user-session,
most session processes (including gnome-terminal-server and unity-
settings-daemon) run *outside* of the logind session/scope and thus not
technically within a PAM session. That's why pam_systemd actually does
start a new full session which makes this slow.

In conclusion: Under an upstart session we never actually had a "full"
new interactive PAM session for the target user anyway, as this was
*already* a PAM session. Thus changing pkexec to common-session-
noninteractive is actually doing (mostly) the same as we previously did
in 16.04 under an upstart session. Thus I am much more convinced now
that switching to common-session-noninteractive is not only correct (and
also fixing this bug), but also not actually that regression prone.

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https://bugs.launchpad.net/bugs/1626651

Title:
  brightness keys are handled slower in Yakkety than Xenial

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