linux-kernel - Re: [PATCH 37/41] rcu: New rcu_user_enter() and rcu_user

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Message-ID: <20120523142213.GF1663@somewhere>
Date:	Wed, 23 May 2012 16:22:17 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 37/41] rcu: New rcu_user_enter() and rcu_user_exit() APIs

On Tue, May 22, 2012 at 11:23:06AM -0700, Paul E. McKenney wrote:
> On Tue, May 01, 2012 at 01:55:11AM +0200, Frederic Weisbecker wrote:
> > These two APIs are provided to help the implementation
> > of an adaptive tickless kernel (cf: nohz cpusets). We need
> > to run into RCU extended quiescent state when we are in
> > userland so that a tickless CPU is not involved in the
> > global RCU state machine and can shutdown its tick safely.
> > 
> > These APIs are called from syscall and exception entry/exit
> > points and can't be called from interrupt.
> > 
> > They are essentially the same than rcu_idle_enter() and
> > rcu_idle_exit() minus the checks that ensure the CPU is
> > running the idle task.
> 
> This looks reasonably sane.  There are a few nits like missing comment
> headers for functions and the need for tracing, but I can handle that
> when I pull it in.  I am happy to do that pretty much any time, but not
> before the API stabilizes.  ;-)
> 
> So let me know when it is ready for -rcu.

Ok. So would you be willing to host this specific part in -rcu? I don't
know if these APIs are welcome upstream if they have no upstream users
yet. OTOH it would be easier for me if I don't need to include these patches
in my endless rebases.

Another solution is to host that in some seperate tree. In yours or in -tip.
Ingo seemed to be willing to host this patchset.

What do you think?

I believe I need to rebase against your latest changes though.
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