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Date: Tue, 22 May 2012 10:16:58 -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 11/41] nohz/cpuset: Don't turn off the tick if rcu needs
it
On Tue, May 01, 2012 at 01:54:45AM +0200, Frederic Weisbecker wrote:
> If RCU is waiting for the current CPU to complete a grace
> period, don't turn off the tick. Unlike dynctik-idle, we
> are not necessarily going to enter into rcu extended quiescent
> state, so we may need to keep the tick to note current CPU's
> quiescent states.
>
> [added build fix from Zen Lin]
Hello, Frederic,
One question below -- why not rcu_needs_cpu() instead of rcu_pending()?
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>
> ---
> include/linux/rcupdate.h | 1 +
> kernel/rcutree.c | 3 +--
> kernel/time/tick-sched.c | 22 ++++++++++++++++++----
> 3 files changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 81c04f4..e06639e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -184,6 +184,7 @@ static inline int rcu_preempt_depth(void)
> extern void rcu_sched_qs(int cpu);
> extern void rcu_bh_qs(int cpu);
> extern void rcu_check_callbacks(int cpu, int user);
> +extern int rcu_pending(int cpu);
> struct notifier_block;
> extern void rcu_idle_enter(void);
> extern void rcu_idle_exit(void);
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 6c4a672..e141c7e 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -212,7 +212,6 @@ int rcu_cpu_stall_suppress __read_mostly;
> module_param(rcu_cpu_stall_suppress, int, 0644);
>
> static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
> -static int rcu_pending(int cpu);
>
> /*
> * Return the number of RCU-sched batches processed thus far for debug & stats.
> @@ -1915,7 +1914,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
> * by the current CPU, returning 1 if so. This function is part of the
> * RCU implementation; it is -not- an exported member of the RCU API.
> */
> -static int rcu_pending(int cpu)
> +int rcu_pending(int cpu)
> {
> return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
> __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 43fa7ac..4f99766 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -506,9 +506,21 @@ void tick_nohz_idle_enter(void)
> local_irq_enable();
> }
>
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +static bool can_stop_adaptive_tick(void)
> +{
> + if (!sched_can_stop_tick())
> + return false;
> +
> + /* Is there a grace period to complete ? */
> + if (rcu_pending(smp_processor_id()))
You lost me on this one. Why can't this be rcu_needs_cpu()?
> + return false;
> +
> + return true;
> +}
> +
> static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts)
> {
> -#ifdef CONFIG_CPUSETS_NO_HZ
> int cpu = smp_processor_id();
>
> if (!cpuset_adaptive_nohz() || is_idle_task(current))
> @@ -517,12 +529,14 @@ static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts)
> if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
> return;
>
> - if (!sched_can_stop_tick())
> + if (!can_stop_adaptive_tick())
> return;
>
> tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
> -#endif
> }
> +#else
> +static void tick_nohz_cpuset_stop_tick(struct tick_sched *ts) { }
> +#endif
>
> /**
> * tick_nohz_irq_exit - update next tick event from interrupt exit
> @@ -852,7 +866,7 @@ void tick_nohz_check_adaptive(void)
> struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
>
> if (ts->tick_stopped && !is_idle_task(current)) {
> - if (!sched_can_stop_tick())
> + if (!can_stop_adaptive_tick())
> tick_nohz_restart_sched_tick();
> }
> }
> --
> 1.7.5.4
>
--
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