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Date: Wed, 12 Sep 2012 15:54:50 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
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        sbw@mit.edu, patches@linaro.org,
        Alessio Igor Bogani <abogani@kernel.org>, Avi Kivity <avi@redhat.com>,
        Chris Metcalf <cmetcalf@tilera.com>, Christoph Lameter <cl@linux.com>,
        Geoff Levand <geoff@infradead.org>,
        Gilad Ben Yossef <gilad@benyossef.com>,
        Hakan Akkan <hakanakkan@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
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        Max Krasnyansky <maxk@qualcomm.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
Subject: Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user
 preemption
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On Wed, Sep 12, 2012 at 02:52:40PM +0200, Peter Zijlstra wrote:
> 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.

I don't know that part of -rt. Probably we can deal with that later once
we have some upstream code in place?

> 
> Bugger it.. I so detest this patch.