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

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Message-ID: <20120912152156.GP4257@linux.vnet.ibm.com>
Date:	Wed, 12 Sep 2012 08:21:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.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, Sep 12, 2012 at 02:41:36PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-09-12 at 14:06 +0200, Frederic Weisbecker wrote:
> > 
> > 1) This can happen if something calls set_need_resched() while no other task is
> > on the runqueue.
> 
> People really shouldn't be doing that... I think I know why RCU does
> this, but yuck. I also think RCU can avoid doing this, but its a toss up
> if that's worth the trouble.

It used to do this whenever it had to force quiescent states on a given
CPU more than one time in a given grace period.  I have removed this
(on your advice), and the only remaining use is during RCU CPU stall
warnings, where a given CPU has refused to pass through a quiescent
state for more than a minute.

I could remove that one as well, if it would help.  Though it would
be nice to have -some- way to kick the CPU in that case.  Thoughts?

> > 2) Remote wake up done but we haven't yet received the schedule IPI.
> > 
> > 3) Non IPI remote wakeup you're referring above, I'm not sure
> > what you mean though.
> 
> Well there's two ways of doing remote wakeups, one is doing the wakeup
> from the waking cpu and sending an IPI over to reschedule iff you need
> wakeup-preemption, the other is queueing the task remotely and sending
> an IPI to do the wakeup on the remote cpu.
> 
> The former has the problem, the latter not. 
> 
> See ttwu_queue().
> 
> 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?

Indeed it never has done this, at least not in the absence of bugs.

							Thanx, Paul

> 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 ?
> --
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