Have you tried to profile something like umount -l on a large mount tree?
You variant causes a shitstorm of
schedule work
switch to workqueue
do actual fs shutdown
wake umount(8) up
get through wait_for_completion()
for every bleeding vfsmount in there. And no, it's *not* guaranteed to
be dominated by fs shutdown time. Here's the case where it definitely
won't be:
mkdir /tmp/a
mount --rbind / /tmp/a
umount -l /tmp/a
All vfsmounts involved are killed off with no fs shutdown. And that's
*not* a rare case - exit of the last process in namespace is very likely
to look that way too.
That's far too heavy.
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