linux-kernel - Re: Does anyone use CONFIG_TINY_PREEMPT

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Message-ID: <20121113013236.GM2518@linux.vnet.ibm.com>
Date:	Mon, 12 Nov 2012 17:32:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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,
	sbw@....edu, patches@...aro.org
Subject: Re: Does anyone use CONFIG_TINY_PREEMPT_RCU?

On Tue, Nov 13, 2012 at 02:12:27AM +0100, Frederic Weisbecker wrote:
> 2012/11/13 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > 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.
> 
> I don't use it personally but if you remove it, does that mean that
> RCU couldn't be preemptible on UP?

It would mean that a kernel built with SMP=n and PREEMPT=y would use
TREE_PREEMPT_RCU rather than the current TINY_PREEMPT_RCU.  So it would
work just as well, but use somewhat more memory.

							Thanx, Paul

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