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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Liam,

On Fri, Sep 15, 2023 at 7:45 PM Liam R. Howlett <Liam.Howlett@oracle.com> 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@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>

Thanks for your patch!

This fixes the

      WARNING: CPU: 0 PID: 0 at init/main.c:992 start_kernel+0x2f0/0x480

I was seeing during boot on Renesas RZ/A1 and RZ/A2 since commit
cfeb6ae8bcb96ccf ("maple_tree: disable mas_wr_append() when other
readers are possible") in v6.5.

And unlike v1, this does not cause lots of new warnings on e.g. R-Car M2-W.

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds