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

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Date:   Wed, 13 Sep 2023 15:52:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Andreas Schwab <schwab@...ux-m68k.org>,
        Matthew Wilcox <willy@...radead.org>,
        Peng Zhang <zhangpeng.00@...edance.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] init/main: Clear boot task idle flag

On Tue, Sep 12, 2023 at 08:56:47PM -0400, Liam R. Howlett wrote:

> 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();
>  

Hmm, isn't that pid-1 you're setting PF_IDLE on?

The task becoming idle is 'current' at this point, see the
cpu_startup_entry() call below.

Would not something like so be the right thing?


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2299a5cfbfb9..802551e0009b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
 	 * PF_KTHREAD should already be set at this point; regardless, make it
 	 * look like a proper per-CPU kthread.
 	 */
-	idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
+	idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;
 	kthread_set_per_cpu(idle, cpu);
 
 #ifdef CONFIG_SMP
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 342f58a329f5..5007b25c5bc6 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -373,6 +373,7 @@ EXPORT_SYMBOL_GPL(play_idle_precise);
 
 void cpu_startup_entry(enum cpuhp_state state)
 {
+	current->flags |= PF_IDLE;
 	arch_cpu_idle_prepare();
 	cpuhp_online_idle(state);
 	while (1)