linux-kernel - Re: [PATCH v3 7/7] cgroup: document cgroup v2 freezer interface

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Message-ID: <20181119174241.GA7273@tower.DHCP.thefacebook.com>
Date:   Mon, 19 Nov 2018 17:42:48 +0000
From:   Roman Gushchin <guro@...com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
CC:     Roman Gushchin <guroan@...il.com>, Tejun Heo <tj@...nel.org>,
        "Oleg Nesterov" <oleg@...hat.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] cgroup: document cgroup v2 freezer interface

On Sat, Nov 17, 2018 at 12:02:28AM -0800, Mike Rapoport wrote:
> Hi,
> 
> On Fri, Nov 16, 2018 at 04:38:30PM -0800, Roman Gushchin wrote:
> > Describe cgroup v2 freezer interface in the cgroup v2 admin guide.
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: linux-doc@...r.kernel.org
> > Cc: kernel-team@...com
> > ---
> >  Documentation/admin-guide/cgroup-v2.rst | 26 +++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > index 184193bcb262..a065c0bed88c 100644
> > --- a/Documentation/admin-guide/cgroup-v2.rst
> > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > @@ -862,6 +862,8 @@ All cgroup core files are prefixed with "cgroup."
> >  	  populated
> >  		1 if the cgroup or its descendants contains any live
> >  		processes; otherwise, 0.
> > +	  frozen
> > +		1 if the cgroup is frozen; otherwise, 0.
> > 
> >    cgroup.max.descendants
> >  	A read-write single value files.  The default is "max".
> > @@ -895,6 +897,30 @@ All cgroup core files are prefixed with "cgroup."
> >  		A dying cgroup can consume system resources not exceeding
> >  		limits, which were active at the moment of cgroup deletion.
> > 
> > +  cgroup.freeze
> > +	A read-write single value file which exists on non-root cgroups.
> > +	Allowed values are "0" and "1". The default is "0".
> > +
> > +	Writing "1" to the file causes freezing of the cgroup and all
> > +	descendant cgroups. This means that all belonging processes will
> > +	be stopped and will not run until the cgroup will be explicitly
> > +	unfrozen. Freezing of the cgroup may take some time; when the process
> 
> "when the process is complete" sounds somewhat ambiguous, it's unclear
> whether freezing is complete or the process that's being frozen is
> complete.
> 
> Maybe "when this action is completed"?
> 
> > +	is complete, the "frozen" value in the cgroup.events control file
> > +	will be updated and the corresponding notification will be issued.
> 
> Can you please clarify how exactly cgroup.events would be updated?
> 
> > +	Cgroup can be frozen either by its own settings, either by settings
> 
>       ^ A cgroup ... and maybe there are more "a" and "the" that should be
> fixed, it's hard for me to tell.
> 
> Also, I believe "either ..., or ..." sounds better than "either ...,
> either ..."
> 
> > +	of any ancestor cgroups. If any of ancestor cgroups is frozen, the
> > +	cgroup will remain frozen.
> > +
> > +	Processes in the frozen cgroup can be killed by a fatal signal.
> > +	They also can enter and leave a frozen cgroup: either by an explicit
> > +	move by a user, either if freezing of the cgroup races with fork().
> 
> ditto
> 
> > +	If a cgroup is moved to a frozen cgroup, it stops. If a process is
> 
>             ^ process?
> 
> > +	moving out of a frozen cgroup, it becomes running.
> 
>        ^ moved

Hello, Mike!

Thanks for the review! I agree with all comments above; fixes queued for v4.

> 
> > +	Frozen status of a cgroup doesn't affect any cgroup tree operations:
> > +	it's possible to delete a frozen (and empty) cgroup, as well as
> > +	create new sub-cgroups.
> 
> Maybe it's also worth adding that freezing a process has no effect on its
> memory consumption, at least directly.

Hm, isn't it the expected behavior?

In any case, I assume that cgroup.freeze knob description is not the best place
for a such explanations. Maybe it's better to add a standalone paragraph with
the description of the frozen process state, what's expected to work, what's
not, etc. I'd return to this question a bit later, when we'll agree on the user
interface and the implementation.

Thanks!