Re: [PATCH] cgroup-v1: freezer: optionally killable freezer

From: Daniel Colascione <dancol@google.com>
To: Tejun Heo <tj@kernel.org>
Cc: Marco Ballesio <balejs@google.com>, Roman Gushchin <guro@fb.com>,
	cgroups@vger.kernel.org,
	linux-kernel <linux-kernel@vger.kernel.org>,
	lizefan@huawei.com, Johannes Weiner <hannes@cmpxchg.org>,
	Jonathan Corbet <corbet@lwn.net>,
	rjw@rjwysocki.net, Pavel Machek <pavel@ucw.cz>,
	len.brown@intel.com,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	linux-pm@vger.kernel.org, Minchan Kim <minchan@google.com>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH] cgroup-v1: freezer: optionally killable freezer
Date: Wed, 11 Mar 2020 10:46:15 -0700	[thread overview]
Message-ID: <CAKOZuevzE=0Oa8gn--rkVJ8t69S+o2vK--pki65XXg6EVuOhMQ@mail.gmail.com> (raw)
In-Reply-To: <20200303134855.GA186184@mtj.thefacebook.com>

On Tue, Mar 3, 2020 at 5:48 AM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Wed, Feb 19, 2020 at 10:32:31AM -0800, Marco Ballesio wrote:
> > @@ -94,6 +94,18 @@ The following cgroupfs files are created by cgroup freezer.
> >    Shows the parent-state.  0 if none of the cgroup's ancestors is
> >    frozen; otherwise, 1.
> >
> > +* freezer.killable: Read-write
> > +
> > +  When read, returns the killable state of a cgroup - "1" if frozen
> > +  tasks will respond to fatal signals, or "0" if they won't.
> > +
> > +  When written, this property sets the killable state of the cgroup.
> > +  A value equal to "1" will switch the state of all frozen tasks in
> > +  the cgroup to TASK_INTERRUPTIBLE (similarly to cgroup v2) and will
> > +  make them react to fatal signals. A value of "0" will switch the
> > +  state of frozen tasks to TASK_UNINTERRUPTIBLE and they won't respond
> > +  to signals unless thawed or unfrozen.
>
> As Roman said, I'm not too sure about adding a new cgroup1 freezer
> interface at this point. If we do this, *maybe* a mount option would
> be more minimal?

I'd still prefer a cgroup flag. A mount option is a bigger
compatibility risk and isn't really any simpler than another cgroup
flag. A mount option will affect anything using the cgroup mount
point, potentially turning non-killable frozen processes into killable
ones unexpectedly. (Sure, you could mount multiple times, but only one
location is canonical, and that's the one that's going to get the flag
flipped.) A per-cgroup flag allows people to opt into the new behavior
only in specific contexts, so it's safer.