From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S964998AbbI2NIL (ORCPT <rfc822;w@1wt.eu>);
	Tue, 29 Sep 2015 09:08:11 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:35416 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S964984AbbI2NIE (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 29 Sep 2015 09:08:04 -0400
Date: Tue, 29 Sep 2015 15:02:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, oleg@redhat.com,
        umgwanakikbuti@gmail.com, tglx@linutronix.de, rostedt@goodmis.org
Subject: Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant
Message-ID: <20150929130201.GH3816@twins.programming.kicks-ass.net>
References: <20150929092825.540553633@infradead.org>
 <20150929093519.817299442@infradead.org>
 <20150929125512.GB31582@lerouge>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150929125512.GB31582@lerouge>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Sep 29, 2015 at 02:55:13PM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 29, 2015 at 11:28:27AM +0200, Peter Zijlstra wrote:
> >  #define init_task_preempt_count(p) do { \
> > -	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
> > +	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
> 
> Since it's not quite obvious why we use this magic value without looking
> at schedule_tail() details, maybe add a little comment? (Just "/* see schedule_tail() */").

Right, I fixed that in 12/11 v2. I'll change that around a bit.

> > +	/*
> > +	 * Still have preempt_count() == 2, from:
> > +	 *
> > +	 *	schedule()
> > +	 *	  preempt_disable();			// 1
> > +	 *	  __schedule()
> > +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> > +	 */
> 
> I found that a bit confusing first, because that's a preempt_count()
> we actually emulate for a new task. Maybe something like:
> 
> +	/*
> +	 * New task is init with preempt_count() == 2 because prev task left
> +        * us after:
> +	 *
> +	 *	schedule()
> +	 *	  preempt_disable();			// 1
> +	 *	  __schedule()
> +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> +	 */

I think I'll move the comment to finish_task_switch(), but yes.