linux-kernel - Re: [PATCH tip/core/rcu 1/9] rcu: Expedite grace periods during suspend/resume

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Date:	Tue, 20 Aug 2013 17:58:40 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Borislav Petkov <bp@...en8.de>,
	Borislav Petkov <bp@...e.de>,
	Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Expedite grace periods during suspend/resume

On 08/20/2013 10:42 AM, Paul E. McKenney wrote:
> From: Borislav Petkov <bp@...en8.de>
> 
> CONFIG_RCU_FAST_NO_HZ can increase grace-period durations by up to
> a factor of four, which can result in long suspend and resume times.
> Thus, this commit temporarily switches to expedited grace periods when
> suspending the box and return to normal settings when resuming.  Similar
> logic is applied to hibernation.
> 
> Because expedited grace periods are of dubious benefit on very large
> systems, so this commit restricts their automated use during suspend
> and resume to systems of 256 or fewer CPUs.  (Some day a number of
> Linux-kernel facilities, including RCU's expedited grace periods,
> will be more scalable, but I need to see bug reports first.)
> 
> [ paulmck: This also papers over an audio/irq bug, but hopefully that will
>   be fixed soon. ]
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> Signed-off-by: Bjørn Mork <bjorn@...k.no>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Reviewed-by: Josh Triplett <josh@...htriplett.org>
> ---
>  kernel/rcutree.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 338f1d1..a7bf517 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -54,6 +54,7 @@
>  #include <linux/stop_machine.h>
>  #include <linux/random.h>
>  #include <linux/ftrace_event.h>
> +#include <linux/suspend.h>
>  
>  #include "rcutree.h"
>  #include <trace/events/rcu.h>
> @@ -3032,6 +3033,25 @@ static int rcu_cpu_notify(struct notifier_block *self,
>  	return NOTIFY_OK;
>  }
>  
> +static int rcu_pm_notify(struct notifier_block *self,
> +			 unsigned long action, void *hcpu)
> +{
> +	switch (action) {
> +	case PM_HIBERNATION_PREPARE:
> +	case PM_SUSPEND_PREPARE:
> +		if (nr_cpu_ids <= 256) /* Expediting bad for large systems. */
> +			rcu_expedited = 1;
> +		break;
> +	case PM_POST_HIBERNATION:
> +	case PM_POST_SUSPEND:
> +		rcu_expedited = 0;

Users can set it via sysfs, this notify will changes it.
I think we can introduce an rcu_expedited_syfs_saved;
thus we can change this line to:
-		rcu_expedited = 0;
+		rcu_expedited = rcu_expedited_syfs_saved;


rcu_init() {
	...
+	rcu_expedited_syfs_saved = rcu_expedited;
}

static ssize_t rcu_expedited_store(struct kobject *kobj,
				   struct kobj_attribute *attr,
				   const char *buf, size_t count)
{
	if (kstrtoint(buf, 0, &rcu_expedited))
		return -EINVAL;

+	rcu_expedited_syfs_saved = rcu_expedited;
	return count;
}

> +		break;
> +	default:
> +		break;
> +	}
> +	return NOTIFY_OK;
> +}
> +
>  /*
>   * Spawn the kthread that handles this RCU flavor's grace periods.
>   */
> @@ -3273,6 +3293,7 @@ void __init rcu_init(void)
>  	 * or the scheduler are operational.
>  	 */
>  	cpu_notifier(rcu_cpu_notify, 0);
> +	pm_notifier(rcu_pm_notify, 0);
>  	for_each_online_cpu(cpu)
>  		rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
>  }

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