linux-kernel - Re: [PATCH tip/core/rcu 04/26] rcu: Settle config for userspace extended quiescent state

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Message-ID: <20120910213506.GC21277@somewhere.redhat.com>
Date:	Mon, 10 Sep 2012 23:35:19 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	sbw@....edu, patches@...aro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH tip/core/rcu 04/26] rcu: Settle config for userspace
 extended quiescent state

On Tue, Sep 04, 2012 at 05:46:19PM -0700, Josh Triplett wrote:
> > It actually does depend on SMP.  There has to be at least one CPU taking
> > scheduling-clock interrupts in order to keep time computation accurate,
> > so a de-facto UP system cannot adaptive-dynticks its sole CPU.
> 
> Ah.  That seems like a removable limitation, albeit a difficult one.
> Nonetheless, it makes sense to avoid providing the option when it won't
> help.
> 
> However, once a config symbol for adaptive dynticks exists, perhaps that
> symbol should depend on SMP and RCU_USER_QS should depend on that
> instead, documenting the limitation in the right place and making it
> easier to find and change eventually.

Right! And in fact CONFIG_RCU_USER_QS is a temporary config. Once we'll have
CONFIG_NO_HZ_FULL, we won't need intermediate configs like this.
And CONFIG_NO_HZ_FULL will depend on SMP anyway.
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