linux-kernel - Re: Does anyone use CONFIG_TINY_PREEMPT

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Message-ID: <20121113175654.GA30119@elliptictech.com>
Date:	Tue, 13 Nov 2012 12:56:54 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: Does anyone use CONFIG_TINY_PREEMPT_RCU?

On 2012-11-13 09:08 -0800, Paul E. McKenney wrote:
> On Tue, Nov 13, 2012 at 09:46:20AM -0500, Nick Bowler wrote:
> > On 2012-11-12 16:49 -0800, Paul E. McKenney wrote:
> > > Hello!
> > > 
> > > I know of people using TINY_RCU, TREE_RCU, and TREE_PREEMPT_RCU, but I
> > > have not heard of anyone using TINY_PREEMPT_RCU for whom TREE_PREEMPT_RCU
> > > was not a viable option (in contrast, the people running Linux on
> > > tiny-memmory systems typically use TINY_RCU).  Of course, if no one
> > > really needs it, the proper thing to do is to remove it.
> > > 
> > > So, if you need TINY_PREEMPT_RCU, please let me know.  Otherwise, I will
> > > remove it, probably in the 3.9 timeframe.
> > 
> > Yes, I use TINY_PREEMPT_RCU on my UP machines.  It is, in fact, the only
> > option.
> 
> Suppose that TREE_PREEMPT_RCU was available for !SMP && PREEMPT builds.
> Would that work for you?

To be honest I don't really know what the difference is, other than what
the help text says, which is:

  [TINY_PREEMPT_RCU] greatly reduces the memory footprint of RCU.
  
"Greatly reduced memory footprint" sounds pretty useful...

As a side note, I wonder why any of these RCU implementations are
user-seclectable options in the first place?  It looks like you will
only ever have one choice, since the dependencies all seem mutually
exclusive:

  TREE_RCU         depends on !PREEMPT &&  SMP
  TREE_PREEMPT_RCU depends on  PREEMPT &&  SMP
  TINY_RCU         depends on !PREEMPT && !SMP
  TINY_PREEMPT_RCU depends on  PREEMPT && !SMP

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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