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SubjectRe: [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@gmail.com>
> Cc: Alessio Igor Bogani <abogani@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> Cc: Geoff Levand <geoff@infradead.org>
> Cc: Gilad Ben Yossef <gilad@benyossef.com>
> Cc: Hakan Akkan <hakanakkan@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Kevin Hilman <khilman@ti.com>
> Cc: Max Krasnyansky <maxk@qualcomm.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.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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