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

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Message-ID: <20200320201038.GB79184@google.com>
Date:   Fri, 20 Mar 2020 13:10:38 -0700
From:   Marco Ballesio <balejs@...gle.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>,
        cgroups@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>, lizefan@...wei.com,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>, rjw@...ysocki.net,
        Pavel Machek <pavel@....cz>, len.brown@...el.com,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-pm@...r.kernel.org, Minchan Kim <minchan@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] cgroup-v1: freezer: optionally killable freezer

On Wed, Mar 11, 2020 at 10:46:15AM -0700, Daniel Colascione wrote:
> On Tue, Mar 3, 2020 at 5:48 AM Tejun Heo <tj@...nel.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.

It might also be desirable for userland to have a way to modify the behavior of
an already mounted v1 freezer.

Tejun, would it be acceptable to have a flag but disable it by default, hiding
it behind a kernel configuration option?