Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Mathieu Desnoyers]

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