From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1762969AbXJENVb@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1762969AbXJENVb (ORCPT <rfc822;w@1wt.eu>);
	Fri, 5 Oct 2007 09:21:31 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755553AbXJENVU
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Fri, 5 Oct 2007 09:21:20 -0400
Received: from e31.co.us.ibm.com ([32.97.110.149]:48011 "EHLO
	e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1762415AbXJENVT (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 5 Oct 2007 09:21:19 -0400
Date: Fri, 5 Oct 2007 18:51:14 +0530
From: Gautham R Shenoy <ego@in.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>,
       linux-rt-users <linux-rt-users@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, akpm@linux-foundation.org,
       dipankar@in.ibm.com, josht@linux.vnet.ibm.com, tytso@us.ibm.com,
       dvhltc@us.ibm.com, Thomas Gleixner <tglx@linutronix.de>,
       a.p.zijlstra@chello.nl, bunk@kernel.org, oleg@tv-sign.ru
Subject: Re: [PATCH RFC 6/9] RCU priority boosting for preemptible RCU
Message-ID: <20071005132114.GA28755@in.ibm.com>
Reply-To: ego@in.ibm.com
References: <20070910183004.GA3299@linux.vnet.ibm.com> <20070910183901.GF3819@linux.vnet.ibm.com> <20071005114657.GA17788@in.ibm.com> <Pine.LNX.4.58.0710050822330.20859@gandalf.stny.rr.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Pine.LNX.4.58.0710050822330.20859@gandalf.stny.rr.com>
User-Agent: Mutt/1.5.12-2006-07-14
Sender: linux-kernel-owner@vger.kernel.org
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Oct 05, 2007 at 08:24:21AM -0400, Steven Rostedt wrote:
> 
> 
> 
> On Fri, 5 Oct 2007, Gautham R Shenoy wrote:
> 
> > On Mon, Sep 10, 2007 at 11:39:01AM -0700, Paul E. McKenney wrote:
> >
> > [snip]
> >
> > > +
> > > +/*
> > > + * Return the list from which to boost target tasks.
> > > + * May only be invoked by the booster task, so guaranteed to
> > > + * already be initialized.  Use rcu_boost_dat element least recently
> > > + * the destination for task blocking in RCU read-side critical sections.
> > > + */
> > > +static inline struct rcu_boost_dat *rcu_rbd_boosting(int cpu)
> > > +{
> > > +	int idx = (rcu_boost_idx + 1) & (RCU_BOOST_ELEMENTS - 1);
> >
> > Why is this masking required? When we increment
> > the rcu_boost_idx in rcu_booster, we do perform a modulo operation
> > to ensure that it wraps around RCU_BOOST_ELEMENTS.
> 
> Because we are not masking rcu_boost_idx, we are masking
>  (rcu_boost_idx + 1) which may extend the bounderies of
> RCU_BOOST_ELEMENTS.

Thanks! 

But I'm still trying to understand why the (increment + masking)
is required at all.

The thread(producer) that requires boosting is added to the element
with index rcu_boost_idx.

The booster thread(consumer) increments the rcu_boost_idx to 
(rcu_boost_idx + 1) % RCU_BOOST_ELEMENTS, before it fetches the least
recently used rcu_boot_dat elements and boost the eligible tasks queued
in that element.

So, can't we just return per_cpu(rcu_boost_dat, cpu)[rcu_boost_idx] from
rcu_rbd_boosting(cpu) ? Isn't that already the least recently used
element?

> 
> -- Steve

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"