linux-kernel - Re: [PATCH v2 3/6] cgroup: cgroup v2 freezer

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Date:   Wed, 14 Nov 2018 17:06:06 +0000
From:   Roman Gushchin <guro@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     Roman Gushchin <guroan@...il.com>, Tejun Heo <tj@...nel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 3/6] cgroup: cgroup v2 freezer

On Wed, Nov 14, 2018 at 05:56:32PM +0100, Oleg Nesterov wrote:
> Hi Roman,
> 
> On 11/13, Roman Gushchin wrote:
> >
> > > > +#define TASK_FROZEN			0x1000
> > > > +#define TASK_STATE_MAX			0x2000
> > >
> > > Just noticed the new task state... Why? Can't we avoid it?
> >
> > We can, but it's nice to show to userspace that tasks are frozen,
> > rather than just stuck somewhere in the kernel...
> 
> But then you need to change get_task_state() too. Which iiuc could
> probably check ->frozen along with ->state.
> 
> I do not think the new task state is a good idea, at least I would like
> to ask you to make a separate patch which we can discuss separately.

Yeah, I have separated it in v3.

> 
> 
> > > > +	set_current_state(TASK_WAKEKILL | TASK_INTERRUPTIBLE | TASK_FROZEN);
> > >
> > > Why not __set_current_state() ?
> >
> > Hm, it's not a hot path at all, so set_current_state() is good enough.
> > Not a strong preference, of course.
> 
> It is not about performance, to me set_current_state() looks as if we need
> a memory barrier for some obscure/undocumented reason and this doesn't help
> to understand the code.
> 
> > > If ->state include TASK_INTERRUPTIBLE, why do we need TASK_WAKEKILL?
> > >
> > > And again, why TASK_FROZEN?
> >
> > So, should it be just TASK_INTERRUPTIBLE | TASK_FROZEN ?
> 
> Again, TASK_FROZEN is pointless at least until you change fs/proc or until
> you have wake_up_state(TASK_FROZEN). May be cgroup_do_freeze() and/or
> ptrace_attach() could use it, but see above, I'd suggest to make another
> patch.
> 
> Looks like you need TASK_KILLABLE, see below.
> 
> > > > +	clear_thread_flag(TIF_SIGPENDING);
> > > > +	schedule();
> > > > +	recalc_sigpending();
> > >
> > > I simply can't understand these 3 lines above but I bet this is not correct ;)
> >
> > So, yeah, the problem is that if there is TIF_SIGPENDING bit set, schedule()
> > will return immediately, so we're getting pretty much a busy loop here.
> 
> I suspected this answer ;)
> 
> > This is a nasty workaround.
> 
> No, this is very wrong. Just suppose the caller is killed right before
> clear_thread_flag(TIF_SIGPENDING).

So, I had TASK_KILLABLE before, but had some issues with ptrace/gdb.
I'll revisit this option.

> 
> > I believe we can clear and not call recalc_sigpending() at all. Does this seem
> > to be correct?
> 
> I think you need to simply remove both clear_thread_flag() and recalc_sigpending().
> If schedule() is called in TASK_KILLABLE state it will return only if
> fatal_signal_pending() is true, and this is what we want, right?
> 
> OK, it seems you are going to make the new version anyway, so I can wait for it
> and not read this series ;)

Sure! I'm about to post it.

Thanks!