Re: [PATCH] init/main: Clear boot task idle flag

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 12, 2023 at 08:56:47PM -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 clearing the PF_IDLE flag on return from sched_init()
> and restore the flag in rest_init().  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@mail.gmail.com/
> Fixes: 5f6130fa52ee ("tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task")
> Cc: stable@vger.kernel.org
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peng Zhang <zhangpeng.00@bytedance.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  init/main.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/init/main.c b/init/main.c
> index ad920fac325c..f74772acf612 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -696,7 +696,7 @@ noinline void __ref __noreturn rest_init(void)
>  	 */
>  	rcu_read_lock();
>  	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
> -	tsk->flags |= PF_NO_SETAFFINITY;
> +	tsk->flags |= PF_NO_SETAFFINITY | PF_IDLE;
>  	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
>  	rcu_read_unlock();
>  
> @@ -938,6 +938,8 @@ void start_kernel(void)
>  	 * time - but meanwhile we still have a functioning scheduler.
>  	 */
>  	sched_init();
> +	/* Avoid early context switch, rest_init() restores PF_IDLE */
> +	current->flags &= ~PF_IDLE;
>  
>  	if (WARN(!irqs_disabled(),
>  		 "Interrupts were enabled *very* early, fixing it\n"))


Hurmph... so since this is about IRQs, would it not make sense to have
the | PF_IDLE near 'early_boot_irqs_disabled = false' ?

Or, alternatively, make the tinyrcu thing check that variable?