Re: [PATCH v2] kernel/sched: Modify initial boot task idle setup

[Peter Zijlstra]

On Fri, Sep 15, 2023 at 01:44:44PM -0400, Liam R. Howlett wrote:
> Initial booting is setting the task flag to idle (PF_IDLE) by the call
> path sched_init() -> init_idle().  Having the task idle and calling
> call_rcu() in kernel/rcu/tiny.c means that TIF_NEED_RESCHED will be
> set.  Subsequent calls to any cond_resched() will enable IRQs,
> potentially earlier than the IRQ setup has completed.  Recent changes
> have caused just this scenario and IRQs have been enabled early.
> 
> This causes a warning later in start_kernel() as interrupts are enabled
> before they are fully set up.
> 
> Fix this issue by setting the PF_IDLE flag later in the boot sequence.
> 
> Although the boot task was marked as idle since (at least) d80e4fda576d,
> I am not sure that it is wrong to do so.  The forced context-switch on
> idle task was introduced in the tiny_rcu update, so I'm going to claim
> this fixes 5f6130fa52ee.
> 
> Link: https://lore.kernel.org/linux-mm/87v8cv22jh.fsf@mail.lhotse/
> Link: https://lore.kernel.org/linux-mm/CAMuHMdWpvpWoDa=Ox-do92czYRvkok6_x6pYUH+ZouMcJbXy+Q@xxxxxxxxxxxxxx/
> Fixes: 5f6130fa52ee ("tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task")
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---

Thanks!, I've queued this up for sched/urgent but will let the robots at
it before I push it out to -tip.