linux-kernel - Re: Does anyone use CONFIG_TINY_PREEMPT

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Message-ID: <20121113014252.GO2518@linux.vnet.ibm.com>
Date:	Mon, 12 Nov 2012 17:42:52 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	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,
	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 02:34:06AM +0100, Frederic Weisbecker wrote:
> 2012/11/13 Josh Triplett <josh@...htriplett.org>:
> > 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?
> >
> > No, it would mean that on UP you could choose between TINY_RCU and
> > TREE_PREEMPT_RCU, depending on whether you want tiny or preemptible.
> 
> Ok. I thought the TREE version wasn't possible anymore on UP when I
> saw some patches that removed optimizations for nr_online_cpus=1.
> Hence the confusion.

Those optimizations are not critically important.  That said, yes, I will
need to restart testing of TREE_PREEMPT_RCU on !SMP kernels.

							Thanx, Paul

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