linux-kernel - Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user preemption

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Message-ID: <1347454360.15764.27.camel@twins>
Date:	Wed, 12 Sep 2012 14:52:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, 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 11/26] rcu: Exit RCU extended QS on user
 preemption

On Wed, 2012-09-12 at 14:41 +0200, Peter Zijlstra wrote:

> We could of course mandate that all remote wakeups to special nohz cpus
> get queued. That would just leave us with RCU and it would simply not
> send resched IPIs to extended quiescent CPUs anyway, right?
> 
> So at that point all return to user schedule() calls have nr_running > 1
> and the tick is running and RCU is not in extended quiescent state.
> Since either we had nr_running > 1 and pre and post state are the same,
> or we had nr_running == 1 and we just got a fresh wakeup pushing it to
> 2, the wakeup will have executed on our cpu and have re-started the tick
> and kicked RCU into active gear again.
> 
> We cannot hit return to user schedule() with nr_running == 0, simply
> because in that case there's no userspace to return to, only the idle
> thread and that's very much not userspace :-)
> 
> Hmm ?

Crap.. this will screw over -rt, since the wakeups batch the IPI can
take forever so we had to disable this.

Bugger it.. I so detest this patch.
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