linux-kernel - Re: [PATCH 4/4] Revert "kernel/sched: Modify initial boot task idle setup"

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Message-ID: <6fdfe084-866a-4ab4-ac03-a8e0e5b5a35e@paulmck-laptop>
Date:   Fri, 20 Oct 2023 08:05:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        Zqiang <qiang.zhang1211@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] Revert "kernel/sched: Modify initial boot task idle
 setup"

On Fri, Oct 20, 2023 at 03:48:36PM +0200, Peter Zijlstra wrote:
> 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.

Booting up kernels being what it is, there might not be a completely
pretty solution.  Not that I would say "no" to such a solution should
it appear, mind you!  ;-)

							Thanx, Paul