linux-kernel - Re: [PATCH 09/32] nohz/cpuset: Restart tick when nohz flag is cleared on cpuset

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Date:	Tue, 30 Oct 2012 14:55:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Alessio Igor Bogani <abogani@...nel.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>,
	Kevin Hilman <khilman@...com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH 09/32] nohz/cpuset: Restart tick when nohz flag is
 cleared on cpuset

On Mon, 2012-10-29 at 16:27 -0400, Steven Rostedt wrote:
> plain text document attachment
> (0009-nohz-cpuset-Restart-tick-when-nohz-flag-is-cleared-o.patch)
> From: Frederic Weisbecker <fweisbec@...il.com>
> 
> Issue an IPI to restart the tick on a CPU that belongs
> to a cpuset when its nohz flag gets cleared.
> 
> 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/cpuset.h   |    2 ++
>  kernel/cpuset.c          |   25 +++++++++++++++++++++++--
>  kernel/time/tick-sched.c |    8 ++++++++
>  3 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 7e7eb41..631968b 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -260,6 +260,8 @@ static inline bool cpuset_adaptive_nohz(void)
>  	 */
>  	return cpuset_cpu_adaptive_nohz(smp_processor_id());
>  }
> +
> +extern void cpuset_exit_nohz_interrupt(void *unused);
>  #else
>  static inline bool cpuset_cpu_adaptive_nohz(int cpu) { return false; }
>  static inline bool cpuset_adaptive_nohz(void) { return false; }
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index 6319d8e..1b67e5b 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -1200,6 +1200,14 @@ static void cpuset_change_flag(struct task_struct *tsk,
>  
>  DEFINE_PER_CPU(atomic_t, cpu_adaptive_nohz_ref);
>  
> +static void cpu_exit_nohz(int cpu)
> +{
> +	preempt_disable();
> +	smp_call_function_single(cpu, cpuset_exit_nohz_interrupt,
> +				 NULL, true);
> +	preempt_enable();
> +}
> +
>  static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
>  {
>  	int cpu;
> @@ -1211,9 +1219,22 @@ static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
>  	for_each_cpu(cpu, cs->cpus_allowed) {
>  		atomic_t *ref = &per_cpu(cpu_adaptive_nohz_ref, cpu);
>  		if (is_adaptive_nohz(cs))
> -			atomic_inc(ref);
> +			val = atomic_inc_return(ref);
>  		else
> -			atomic_dec(ref);
> +			val = atomic_dec_return(ref);
> +
> +		if (!val) {
> +			/*
> +			 * The update to cpu_adaptive_nohz_ref must be
> +			 * visible right away. So that once we restart the tick
> +			 * from the IPI, it won't be stopped again due to cache
> +			 * update lag.
> +			 * FIXME: We probably need more to ensure this value is really
> +			 * visible right away.

What more do you want? stomp_machine()??

> +			 */
> +			smp_mb();

The atomic_inc_return() and atomic_dec_return() already imply a
smp_mb().

Later patches change this code, so I wont dwell on this patch too much.


> +			cpu_exit_nohz(cpu);
> +		}
>  	}
>  }
>  #else
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 0a5e650..de7de68 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -884,6 +884,14 @@ void tick_nohz_check_adaptive(void)
>  	}
>  }
>  
> +void cpuset_exit_nohz_interrupt(void *unused)
> +{
> +	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> +
> +	if (ts->tick_stopped && !is_idle_task(current))
> +		tick_nohz_restart_adaptive();

BTW, what a confusing name. "restart_adaptive()"? It sounds like we are
going to restart the adaptive code, like restarting NOHZ.

-- Steve

> +}
> +
>  void tick_nohz_post_schedule(void)
>  {
>  	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);


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