A "kill" button for control groups

So, regarding use of freezer in systemd for killing: the cgroupsv1 freeze is really broken API-wise. i.e. you need to have a timed sleep loop to see when it is done. It's not usable for any code that strives to be reasonably clean. We never supported it in systemd for anything for this reason. I mean, there are limits to everything how much ugly low-level code we are willing to accept...

The cgroupsv2 freezer makes a ton more sense, and we expose it with hence high level operations (systemctl freeze + systemctl thaw), but we don't use it to make killing race-free. We could do that, but it doesn't feel ideal to me, since freezing is slow, i.e. we need to initiate the freeze, then wait until the kernel tells us it is done (poll()), then enqueue the signal, and then unfreeze and wait again. And blocking syscalls can delay the freeze for long times. Thus killing would become a "slow" operation in the worst case (at least that's my understanding), and that kinda sucks. After all we want this as a clean-up operation that gets rid of broken stuff, i.e. SIGKILL is the unfriendly way to abort stuff, but if things are not abortive anymore if we use the freezer, that defeats half the point.

I love Christian's work on this, since it fixes the race for us *and* is always a quick operation. We don't have to wait for anything "slow". (I mean, it internally also iterates through all processes, so it's not O(1), but that's not what I mean by "slow"...) It just enqueues the SIGKILL for each process in a race-free fashion, and that's all we need.

So, yeah, I am looking forward to Christian' work land and we'll happily make use of it in systemd once it lands. It fixes a real problem for us.

Lennart