linux-kernel - Re: [PATCH v6 2/3] sched: Move task_mm_cid

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Message-ID: <1a295a1e-08da-4684-81be-9539773a1c94@efficios.com>
Date: Thu, 13 Feb 2025 08:55:49 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Gabriele Monaco <gmonaco@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, aubrey.li@...ux.intel.com, yu.c.chen@...el.com,
 Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

On 2025-02-13 08:25, Gabriele Monaco wrote:
> On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
>> kernel test robot noticed
>> "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
>>
>> [    2.640924][    T0] ------------[ cut here ]------------
>> [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
>> __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
>> [    2.642874][    T0] Modules linked in:
>> [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
>> 6.14.0-rc2-00002-g287adf9e9c1f #1
>> [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
>> PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
>> (kernel/workqueue.c:2495 (discriminator 9))
> 
> There seem to be major problems with this configuration, I'm trying to
> understand what's wrong but, for the time being, this patchset is not
> ready for inclusion.

I think there is an issue with the order of init functions at boot.

poking_init() calls mm_alloc(), which ends up calling mm_init().

The WARN_ON() is about a NULL wq pointer, which I suspect happens
if poking_init() is called before workqueue_init_early(), which
allocates system_wq.

Indeed, in start_kernel(), poking_init() is called before
workqueue_init_early().

I'm not sure what are the init order dependencies across subsystems here.
There is the following order in start_kernel():

[...]
         mm_core_init();
         poking_init();
         ftrace_init();

         /* trace_printk can be enabled here */
         early_trace_init();

         /*
          * Set up the scheduler prior starting any interrupts (such as the
          * timer interrupt). Full topology setup happens at smp_init()
          * time - but meanwhile we still have a functioning scheduler.
          */
         sched_init();

         if (WARN(!irqs_disabled(),
                  "Interrupts were enabled *very* early, fixing it\n"))
                 local_irq_disable();
         radix_tree_init();
         maple_tree_init();

         /*
          * Set up housekeeping before setting up workqueues to allow the unbound
          * workqueue to take non-housekeeping into account.
          */
         housekeeping_init();

         /*
          * Allow workqueue creation and work item queueing/cancelling
          * early.  Work item execution depends on kthreads and starts after
          * workqueue_init().
          */
         workqueue_init_early();
[...]

So either we find a way to reorder this, or we make sure poking_init()
does not require the workqueue.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com