linux-kernel - Re: [PATCH 2/2] cgroup-v2/freezer: Print information about unfreezable process

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Message-ID: <aVK8w5vjzf384-m1@slm.duckdns.org>
Date: Mon, 29 Dec 2025 07:39:15 -1000
From: Tejun Heo <tj@...nel.org>
To: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cgroup-v2/freezer: Print information about
 unfreezable process

Hello,

On Mon, Dec 29, 2025 at 01:32:11PM +0800, Pavel Tikhomirov wrote:
> > This is only suitable for debugging, and, for that, this can be done from
> > userspace by walking the tasks and check /proc/PID/wchan. Should be
> > do_freezer_trap for everything frozen. If something is not, read and dump
> > its /proc/PID/stack. Wouldn't that work?
> 
> Yes, I think that will do, Thanks.
> 
> Though the trace printing in /proc/PID/stack is a bit less informative
> than show_stack(), e.g. for my test module
> (https://github.com/Snorch/proc-hang-module) the stack in /proc/PID/stack
> will be just empty. (I guess it has to do with the fact that you need less
> privileges to read /proc/PID/stack than dmesg, so you can do a more
> informative thing in dmesg).

You can use drgn, bpftrace or any other bpf tools to read way more detailed
backtraces.

Thanks.

-- 
tejun