Re: [PATCH v6 4/7] cgroup: cgroup v2 freezer

[Roman Gushchin]

Hi Oleg!

Thank you for looking into the patchset!

Yes, I think you're right: cgroup_exit() should check CGRP_FREEZE bit,
not CGRP_FROZEN. Like cgroup_post_fork() does (a one-liner change below).

About spurious transitions (like frozen->non frozen->frozen on a task
being SIGKILLed):
in early versions of the patchset I've tried to avoid them, but then
following the Tejun's advice
switched over to expose them to a user. The logic behind is simple: if
the state of the cgroup has been changed (a task is gone, for
example), let's notify a user.

nr_tasks_to_freeze is simple a number of non-kthead tasks in the
cgroup, which we do
keep actual only for freezing cgroups (to avoid an unnecessary overhead).

Thanks!

--

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 3e6a7c19bfaf..08c84d2145eb 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5832,7 +5832,7 @@ void cgroup_exit(struct task_struct *tsk)
                css_set_move_task(tsk, cset, NULL, false);
                cset->nr_tasks--;

-               if (unlikely(test_bit(CGRP_FROZEN, &cset->dfl_cgrp->flags))) {
+               if (unlikely(cgroup_task_freeze(tsk))) {
                        /*
                         * Task frozen bit should be cleared at this moment,
                         * and nr_frozen_task should be decreased.

ÐÑ, 25 ÑÐÐ. 2019 Ð. Ð 04:27, Oleg Nesterov <oleg@xxxxxxxxxx>:
>
> Sorry, this version raced with my vacation, I missed it.
>
> I'll try to read this code carefully but after a quick glance I have some
> concerns,
>
> On 12/21, Roman Gushchin wrote:
> >
> > +static void cgroup_update_frozen(struct cgroup *cgrp)
> > +{
> > +     bool frozen;
> > +
> > +     lockdep_assert_held(&css_set_lock);
> > +
> > +     /*
> > +      * If the cgroup has to be frozen (CGRP_FREEZE bit set),
> > +      * and all tasks are frozen or stopped, let's consider
> > +      * the cgroup frozen. Otherwise it's not frozen.
> > +      */
> > +     frozen = test_bit(CGRP_FREEZE, &cgrp->flags) &&
> > +             cgrp->freezer.nr_frozen_tasks +
> > +             cgrp->freezer.nr_stopped_tasks ==
> > +             cgrp->freezer.nr_tasks_to_freeze;
>
> OK. Suppose that cgroup is frozen, CGRP_FROZEN is set, stopped == 0,
> to_freeze = frozen.
>
> One of the task is killed, it calls leave_frozen(). If I read this code path
> correctly, only ->nr_frozen_tasks will be decremented, so "frozen" will be
> "false" when cgroup_update_frozen() is called.
>
> Doesn't this mean that this cgroup will no longer be CGRP_FROZEN even after
> the killed task goes away completely?
>
>
> Or. Suppose that another process picks a task from the CGRP_FROZEN cgroup and
> does PTRACE_ATTACH + PTRACE_INTERRUPT. IIUC, the tracee will only increment
> ->nr_stopped_tasks, it won't touch other counters. Again, cgroup won't be FROZEN
> until PTRACE_CONT'ed tracee does cgroup_leave_stopped() ? This looks strange at
> least.
>
>
>
> SIGSTOP. IIUC, a frozen task sleeping in do_freezer_trap() won't stop. However if
> another thread has already called do_signal_stop(), the woken frozen task will
> react to JOBCTL_STOP_PENDING and stop. And do_signal_stop()->cgroup_enter_stopped()
> will "destroy" CGRP_FROZEN too, or I am totally confused.
>
> OTOH, if you freeze a TASK_STOPPED task's cgroup, this task can react to SIGCONT,
> notify its parent, then freeze again. This is fine, but iiuc this cgroup won't be
> FROZEN in between, cgroup_file_notify() will be called twice...
>
> Oleg.
>