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

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Message-ID: <20150421205218.GB2898@worktop.Skamania.guest>
Date:	Tue, 21 Apr 2015 22:52:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Ingo Molnar <mingo@...nel.org>, Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/4] context_tracking: Inherit TIF_NOHZ through forks
 instead of context switches

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?
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