Re: [PATCH 2/4] context_tracking: Inherit TIF_NOHZ through forks instead of context switches

[Frederic Weisbecker <fweisbec@gmail.com>]

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.