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

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Message-ID: <20181113154349.GF2509588@devbig004.ftw2.facebook.com>
Date:   Tue, 13 Nov 2018 07:43:49 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Roman Gushchin <guroan@...il.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH v2 3/6] cgroup: cgroup v2 freezer

Hello, Oleg.

On Tue, Nov 13, 2018 at 04:37:01PM +0100, Oleg Nesterov wrote:
> On 11/12, Roman Gushchin wrote:
> >
> > This patch implements freezer for cgroup v2. However the functionality
> > is similar, the interface is different to cgroup v1: it follows
> > cgroup v2 interface principles.
> 
> Oh, it seems that I actually need to apply this patch to (try to) understand
> the details ;) Will try tomorrow.

Yeah, it's a bit of a head spin like everything in signal delivery /
ptrace paths.

...
> OK, please forget for now, but perhaps it would be more clean to add
> JOBCTL_TRAP_FREEZE to the JOBCTL_PENDING_MASK check in recalc_sigpending()
> and change get_signal to check JOBCTL_TRAP_MASK | JOBCTL_TRAP_FREEZE; and
> I am not even sure cgroup_freezer_enter() should live in do_jobctl_trap().

I'm sure you're aware of the context but just to refresh - one thing
which was really broken about cgroup1 freezer was that it piggybacked
on hibernation freezer and put frozen tasks in a state which is
undefined when seen from userspace - they're just stuck in D sleep
somewhere in the kernel.  That's fine when the whole system is not
gonna be running, but not when only a subportion is being frozen.

So, the primary goal of cgroup2 freezer is putting the tasks in an
equivalent state as jobctl stop.  It's a jobctl stop but controlled by
cgroup frozen state, meaning that they can be killed, PTRACE_SEIZE'd
and INTERRUPT'ed (PTRACE_ATTACH doesn't work as signal delivery should
be blocked but that's fine) and so on.

Thanks.

-- 
tejun