Re: [PATCH -v8 01/11] CPU hotplug: Provide a generic helper to disable/enable CPU hotplug

[Robin Holt <holt@sgi.com>]

Thank you.  That fixed the alpha allmodconfig case for me.  I am currently
rebuilding all the arm defconfig files and will resubmit -v10 when that
is done.

Robin

On Sat, May 11, 2013 at 02:43:20PM +0530, Srivatsa S. Bhat wrote:
> On 05/11/2013 09:46 AM, Robin Holt wrote:
> > On Fri, May 10, 2013 at 10:03:24AM -0700, Andrew Morton wrote:
> >> On Fri, 10 May 2013 13:11:51 +0200 "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
> >>
> >>> ...
> >>>
> >>>> cpu_hotplug_disable() doesn't get compiled unless we've defined
> >>>> CONFIG_PM_SLEEP_SMP.  I cannot begin to imagine what the logic is
> >>>> behind that!
> >>>
> >>> I suppose it was only used by suspend/hibernate code paths when this was
> >>> introduced.
> >>
> >> OK, well I suspect that what I have now is simply wrong for Robin's
> >> application.  Robin, can you please check this?  We probably want to
> >> make the does-something version of cpu_hotplug_disable/enable available
> >> if CONFIG_HOTPLUG_CPU.
> > 
> > This patch came from "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
> > I think I need to defer.
> 
> Here is a revised patch, which should address all the problems, IMHO.
> Let me know if you face any issues. (And thanks a lot for fixing up the
> !SMP case in the previous version.)
> 
> --------------------------------------------------------------------->
> 
> From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> Subject: [PATCH] CPU hotplug: Provide a generic helper to disable/enable CPU hotplug
> 
> There are instances in the kernel where we would like to disable
> CPU hotplug (from sysfs) during some important operation. Today
> the freezer code depends on this and the code to do it was kinda
> tailor-made for that.
> 
> Restructure the code and make it generic enough to be useful for
> other usecases too.
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> ---
> 
>  include/linux/cpu.h |    4 ++++
>  kernel/cpu.c        |   55 +++++++++++++++++++++------------------------------
>  2 files changed, 27 insertions(+), 32 deletions(-)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index c6f6e08..9f3c7e8 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -175,6 +175,8 @@ extern struct bus_type cpu_subsys;
>  
>  extern void get_online_cpus(void);
>  extern void put_online_cpus(void);
> +extern void cpu_hotplug_disable(void);
> +extern void cpu_hotplug_enable(void);
>  #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
>  #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
>  #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
> @@ -198,6 +200,8 @@ static inline void cpu_hotplug_driver_unlock(void)
>  
>  #define get_online_cpus()	do { } while (0)
>  #define put_online_cpus()	do { } while (0)
> +#define cpu_hotplug_disable()	do { } while (0)
> +#define cpu_hotplug_enable()	do { } while (0)
>  #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
>  /* These aren't inline functions due to a GCC bug. */
>  #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index b5e4ab2..198a388 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -133,6 +133,27 @@ static void cpu_hotplug_done(void)
>  	mutex_unlock(&cpu_hotplug.lock);
>  }
>  
> +/*
> + * Wait for currently running CPU hotplug operations to complete (if any) and
> + * disable future CPU hotplug (from sysfs). The 'cpu_add_remove_lock' protects
> + * the 'cpu_hotplug_disabled' flag. The same lock is also acquired by the
> + * hotplug path before performing hotplug operations. So acquiring that lock
> + * guarantees mutual exclusion from any currently running hotplug operations.
> + */
> +void cpu_hotplug_disable(void)
> +{
> +	cpu_maps_update_begin();
> +	cpu_hotplug_disabled = 1;
> +	cpu_maps_update_done();
> +}
> +
> +void cpu_hotplug_enable(void)
> +{
> +	cpu_maps_update_begin();
> +	cpu_hotplug_disabled = 0;
> +	cpu_maps_update_done();
> +}
> +
>  #else /* #if CONFIG_HOTPLUG_CPU */
>  static void cpu_hotplug_begin(void) {}
>  static void cpu_hotplug_done(void) {}
> @@ -541,36 +562,6 @@ static int __init alloc_frozen_cpus(void)
>  core_initcall(alloc_frozen_cpus);
>  
>  /*
> - * Prevent regular CPU hotplug from racing with the freezer, by disabling CPU
> - * hotplug when tasks are about to be frozen. Also, don't allow the freezer
> - * to continue until any currently running CPU hotplug operation gets
> - * completed.
> - * To modify the 'cpu_hotplug_disabled' flag, we need to acquire the
> - * 'cpu_add_remove_lock'. And this same lock is also taken by the regular
> - * CPU hotplug path and released only after it is complete. Thus, we
> - * (and hence the freezer) will block here until any currently running CPU
> - * hotplug operation gets completed.
> - */
> -void cpu_hotplug_disable_before_freeze(void)
> -{
> -	cpu_maps_update_begin();
> -	cpu_hotplug_disabled = 1;
> -	cpu_maps_update_done();
> -}
> -
> -
> -/*
> - * When tasks have been thawed, re-enable regular CPU hotplug (which had been
> - * disabled while beginning to freeze tasks).
> - */
> -void cpu_hotplug_enable_after_thaw(void)
> -{
> -	cpu_maps_update_begin();
> -	cpu_hotplug_disabled = 0;
> -	cpu_maps_update_done();
> -}
> -
> -/*
>   * When callbacks for CPU hotplug notifications are being executed, we must
>   * ensure that the state of the system with respect to the tasks being frozen
>   * or not, as reported by the notification, remains unchanged *throughout the
> @@ -589,12 +580,12 @@ cpu_hotplug_pm_callback(struct notifier_block *nb,
>  
>  	case PM_SUSPEND_PREPARE:
>  	case PM_HIBERNATION_PREPARE:
> -		cpu_hotplug_disable_before_freeze();
> +		cpu_hotplug_disable();
>  		break;
>  
>  	case PM_POST_SUSPEND:
>  	case PM_POST_HIBERNATION:
> -		cpu_hotplug_enable_after_thaw();
> +		cpu_hotplug_enable();
>  		break;
>  
>  	default:
>