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Date:   Fri, 20 Oct 2023 15:48:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@amd.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Uladzislau Rezki <urezki@gmail.com>, rcu <rcu@vger.kernel.org>,
        Zqiang <qiang.zhang1211@gmail.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Sebastian Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 4/4] Revert "kernel/sched: Modify initial boot task idle
 setup"
Message-ID: <20231020134836.GE33965@noisy.programming.kicks-ass.net>
References: <20231019233543.1243121-1-frederic@kernel.org>
 <20231019233543.1243121-5-frederic@kernel.org>
 <20231020082531.GS33217@noisy.programming.kicks-ass.net>
 <ZTJzEVkVfSMGumWU@lothringen>
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On Fri, Oct 20, 2023 at 02:31:13PM +0200, Frederic Weisbecker wrote:

> Yeah I can't say I really like the old coverage of PF_IDLE either. The new one
> (after Liam's patch) is only halfway better defined though: it makes the boot
> CPU's idle behave quite well: PF_IDLE is set on idle entry. And secondary
> CPU's idle behave quite well also except when they go offline and then online
> again. And then the secondary boot code becomes PF_IDLE.

Bah offline, yeah, we should just not do that :-)

> We probably need something like this:
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 3b9d5c7eb4a2..b24d7937b989 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1394,7 +1394,9 @@ void cpuhp_report_idle_dead(void)
>  {
>  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>  
> +	current->flags &= ~PF_IDLE;
>  	BUG_ON(st->state != CPUHP_AP_OFFLINE);
> +
>  	rcutree_report_cpu_dead();
>  	st->state = CPUHP_AP_IDLE_DEAD;
>  	/*
> @@ -1642,6 +1644,8 @@ void cpuhp_online_idle(enum cpuhp_state state)
>  {
>  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>  
> +	current->flags |= PF_IDLE;
> +
>  	/* Happens for the boot cpu */
>  	if (state != CPUHP_AP_ONLINE_IDLE)
>  		return;

Yeah that works I suppose.