linux-kernel - Re: [PATCH 17/41] rcu: Restart tick if we enqueue a callback in a nohz/cpuset CPU

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Date:	Tue, 22 May 2012 10:27:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 17/41] rcu: Restart tick if we enqueue a callback in a
 nohz/cpuset CPU

On Tue, May 01, 2012 at 01:54:51AM +0200, Frederic Weisbecker wrote:
> If we enqueue an rcu callback, we need the CPU tick to stay
> alive until we take care of those by completing the appropriate
> grace period.
> 
> Thus, when we call_rcu(), send a self IPI that checks rcu_needs_cpu()
> so that we restore a periodic tick behaviour that can take care of
> everything.

Ouch, I hadn't considered RCU callbacks being posted from within an
extended quiescent state.  I guess I need to make __call_rcu() either
complain about this or handle it correctly...  It would -usually- be
harmless, but there is getting to be quite a bit of active machinery
in the various idle loops, so just assuming that it cannot happen is
probably getting to be an obsolete assumption.

							Thanx, Paul

> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Alessio Igor Bogani <abogani@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Avi Kivity <avi@...hat.com>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Geoff Levand <geoff@...radead.org>
> Cc: Gilad Ben Yossef <gilad@...yossef.com>
> Cc: Hakan Akkan <hakanakkan@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Kevin Hilman <khilman@...com>
> Cc: Max Krasnyansky <maxk@...lcomm.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/rcutree.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 3fffc26..b8d300c 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1749,6 +1749,13 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
>  	else
>  		trace_rcu_callback(rsp->name, head, rdp->qlen);
> 
> +	/* Restart the timer if needed to handle the callbacks */
> +	if (cpuset_adaptive_nohz()) {
> +		/* Make updates on nxtlist visible to self IPI */
> +		barrier();
> +		smp_cpuset_update_nohz(smp_processor_id());
> +	}
> +
>  	/* If interrupts were disabled, don't dive into RCU core. */
>  	if (irqs_disabled_flags(flags)) {
>  		local_irq_restore(flags);
> -- 
> 1.7.5.4
> 

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