Re: [PATCH v3 1/5] cgroup: introduce cgroup.kill

On Mon, May 10, 2021 at 09:39:02AM -0600, Jonathan Corbet wrote:

Christian Brauner [off-list ref] writes:

quoted

Introduce the cgroup.kill file. It does what it says on the tin and
allows a caller to kill a cgroup by writing "1" into cgroup.kill.
The file is available in non-root cgroups.

So I feel like I'm missing something fundamental here...perhaps somebody
can supply a suitable cluebat.  It seems inevitable to me that, sooner
or later, somebody will surely wish that this mechanism could send a
signal other than SIGKILL, but this interface won't allow that.  Even if
you won't want to implement an arbitrary signal now, it seems like
defining the interface to require writing "9" rather than "1" would
avoid closing that option off in the future.

I assume there's some reason why you don't want to do that, but I'm to
slow to figure out what it is...?

I think the consensus was that most signals are a process/thread-level thing
and are rarely sent to more than one process, so sending them to all processes
in a cgroup is never required. So far nobody came with any real-life use case
behind SIGKILL. SIGSTOP/SIGCONT could be that case, but the cgroup v2 freezer
is doing virtually the same and eliminates this need.

Even SIGTERM which is often used before SIGKILL as a grateful termination
attempt has to be sent to a proper process in the cgroup (e.g. container-level
init or the main process in the workload).

If a new case will arise, it will be possible to introduce a new interface
file, but it looks very unlikely that there will be many such cases to justify
a more generic interface. On the other side keeping it boolean makes it more
consistent with the rest of the cgroup v2 api.

Thanks!