linux-kernel - Re: [patch] x86: avoid unnecessary smp alternatives switch during suspend/resume

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Date:	Sun, 21 Nov 2010 20:27:41 +1100
From:	Nigel Cunningham <lkml@...elcunningham.com.au>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [patch] x86: avoid unnecessary smp alternatives switch during
 suspend/resume

Hi.

On 20/11/10 11:09, Suresh Siddha wrote:
> During suspend, we disable all the non boot cpus. And during resume we bring
> them all back again. So no need to do alternatives_smp_switch() in between.
>
> This speeds up both suspend and resume paths.
>
> Signed-off-by: Suresh Siddha<suresh.b.siddha@...el.com>
> ---
>
>   arch/x86/kernel/smpboot.c |    7 ++++++-
>   include/linux/cpu.h       |    2 ++
>   kernel/cpu.c              |    3 +++
>   3 files changed, 11 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index f0a0624..0b04ca3 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1349,7 +1349,12 @@ void native_cpu_die(unsigned int cpu)
>   			if (system_state == SYSTEM_RUNNING)
>   				pr_info("CPU %u is now offline\n", cpu);
>
> -			if (1 == num_online_cpus())
> +			/*
> +			 * Don't do the smp alternatives switch during
> +			 * suspend. We will be back in the SMP mode after
> +			 * resume.
> +			 */
> +			if (1 == num_online_cpus()&&  !pm_sleep_smp)
>   				alternatives_smp_switch(0);
>   			return;
>   		}
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 4823af6..8cab04c 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -169,11 +169,13 @@ static inline void cpu_hotplug_driver_unlock(void)
>
>   #ifdef CONFIG_PM_SLEEP_SMP
>   extern int suspend_cpu_hotplug;
> +extern int pm_sleep_smp;
>
>   extern int disable_nonboot_cpus(void);
>   extern void enable_nonboot_cpus(void);
>   #else /* !CONFIG_PM_SLEEP_SMP */
>   #define suspend_cpu_hotplug	0
> +#define pm_sleep_smp		0
>
>   static inline int disable_nonboot_cpus(void) { return 0; }
>   static inline void enable_nonboot_cpus(void) {}
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 8615aa6..2eed810 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -381,6 +381,7 @@ out:
>
>   #ifdef CONFIG_PM_SLEEP_SMP
>   static cpumask_var_t frozen_cpus;
> +int pm_sleep_smp;
>
>   int disable_nonboot_cpus(void)
>   {
> @@ -393,6 +394,7 @@ int disable_nonboot_cpus(void)
>   	 * with the userspace trying to use the CPU hotplug at the same time
>   	 */
>   	cpumask_clear(frozen_cpus);
> +	pm_sleep_smp = 1;
>
>   	printk("Disabling non-boot CPUs ...\n");
>   	for_each_online_cpu(cpu) {
> @@ -454,6 +456,7 @@ void __ref enable_nonboot_cpus(void)
>
>   	cpumask_clear(frozen_cpus);
>   out:
> +	pm_sleep_smp = 0;
>   	cpu_maps_update_done();
>   }
We have a few others things that want to modify their behaviour 
according to whether we're doing the atomic copy/restore. Perhaps it 
would be an idea to just use a single flag, perhaps a value for 
system_state?

Nigel
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