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Date: Tue, 21 Apr 2015 23:06:04 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Chris Metcalf <cmetcalf@ezchip.com>, Ingo Molnar <mingo@kernel.org>,
        Dave Jones <davej@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Rik van Riel <riel@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH 2/4] context_tracking: Inherit TIF_NOHZ through forks
 instead of context switches
Message-ID: <20150421210603.GB10289@lerouge>
References: <1429618989-11104-1-git-send-email-fweisbec@gmail.com>
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 <20150421152600.GE3182@worktop.meeting.verilan.com>
 <20150421165152.GA10289@lerouge>
 <20150421205218.GB2898@worktop.Skamania.guest>
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On Tue, Apr 21, 2015 at 10:52:18PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 21, 2015 at 06:51:54PM +0200, Frederic Weisbecker wrote:
> > On Tue, Apr 21, 2015 at 05:26:00PM +0200, Peter Zijlstra wrote:
> > > On Tue, Apr 21, 2015 at 02:23:07PM +0200, Frederic Weisbecker wrote:
> > > > +void __init context_tracking_cpu_set(int cpu)
> > > >  {
> > > > +	static __initdata bool initialized = false;
> > > > +	struct task_struct *p, *t;
> > > > +
> > > > +	if (!per_cpu(context_tracking.active, cpu)) {
> > > > +		per_cpu(context_tracking.active, cpu) = true;
> > > > +		static_key_slow_inc(&context_tracking_enabled);
> > > > +	}
> > > > +
> > > > +	if (initialized)
> > > > +		return;
> > > > +
> > > > +	set_tsk_thread_flag(&init_task, TIF_NOHZ);
> > > > +
> > > > +	/*
> > > > +	 * There shouldn't be any thread at this early boot stage
> > > > +	 * but the scheduler is ready to host any. So lets walk
> > > > +	 * the tasklist just in case. tasklist_lock isn't necessary
> > > > +	 * either that early but take it for correctness checkers.
> > > > +	 */
> > > > +	read_lock(&tasklist_lock);
> > > > +	for_each_process_thread(p, t)
> > > > +		set_tsk_thread_flag(t, TIF_NOHZ);
> > > 
> > > If there should not be any task, should there not be a WARN_ON_ONCE()
> > > here?
> > 
> > Well, it's legal to have a task at that time because sched_init() was called.
> > I just haven't observed any task other than init/0. But future code (or alternate
> > configs than mine) might create a task between sched_init() and tick_init(). And
> > the above code takes care of such a possibility.
> 
> So why not do it sooner?

Because I need the tick nohz intialization to be after irq initialization (init_IRQ()),
which it depends on.