linux-kernel - Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches

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Message-ID: <20150403171946.GA13598@lerouge>
Date:	Fri, 3 Apr 2015 19:19:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks
 instead of context switches

On Thu, Apr 02, 2015 at 08:08:03PM +0200, Oleg Nesterov wrote:
> On 04/02, Frederic Weisbecker wrote:
> >
> > +void context_tracking_cpu_set(int cpu)
> >  {
> > -	clear_tsk_thread_flag(prev, TIF_NOHZ);
> > -	set_tsk_thread_flag(next, TIF_NOHZ);
> > +	static bool initialized = false;
> > +	struct task_struct *p, *t;
> > +	unsigned long flags;
> > +
> > +	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_irqsave(&tasklist_lock, flags);
> > +	for_each_process_thread(p, t)
> > +		set_tsk_thread_flag(t, TIF_NOHZ);
> > +	read_unlock_irqrestore(&tasklist_lock, flags);
> > +
> > +	initialized = true;
> >  }
> 
> Agreed, but _irqsave is not needed. read_lock(tasklist) should work
> just fine.

Indeed, that was just in case we get more callers but then if it happens,
it's their responsibility to fix the callee. So I'll just change that.

> 
> Any reason 3/3 comes as a separate change? I won't argue, just curious.

Nope, I should fold them indeed.

Thanks.

> 
> Oleg.
> 
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