[RFC,tip/core/rcu,6/7] driver-core/cpu: Add cpu_is_hotpluggable() for rcutorture error analysis

On Sat, Dec 03, 2011 at 10:34:41AM -0800, Paul E. McKenney wrote:
> The rcutorture test now can automatically exercise CPU hotplug and
> collect success statistics, which can be correlated with other rcutorture
> activity.  This permits rcutorture to completely exercise RCU regardless
> of what sort of userspace and filesystem layout is in use.  Unfortunately,
> rcutorture is happy to attempt to offline CPUs that cannot be offlined,
> for example, CPU 0 in both the x86 and ARM architectures.  Although this
> allows rcutorture testing to proceed normally, it confounds attempts at
> error analysis due to the resulting flood of spurious CPU-hotplug errors.
> 
> Therefore, this commit creates a cpu_is_hotpluggable() function that
> allows rcutorture to avoid attempting to offline CPUs that are not
> hotpluggable, which in turn allows rcutorture to avoid reporting spurious
> CPU-hotplug errors.  Note that this function is EXPORT_SYMBOL_GPL()
> to allow rcutorture to use it when compiled as a kernel module.
> This commit also includes modifications to rcutorture to use this
> new function.

I'd suggest writing this as a two-patch series: add the API, then use
it.

I'd also suggest making the more general case for this API, beyond just
rcutorture.

> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -23,6 +23,7 @@ struct sysdev_class cpu_sysdev_class = {
>  EXPORT_SYMBOL(cpu_sysdev_class);
>  
>  static DEFINE_PER_CPU(struct sys_device *, cpu_sys_devices);
> +static DEFINE_PER_CPU(bool, is_hotpluggable);
[...]
> @@ -224,8 +226,10 @@ int __cpuinit register_cpu(struct cpu *cpu, int num)
>  
>  	error = sysdev_register(&cpu->sysdev);
>  
> -	if (!error && cpu->hotpluggable)
> +	if (!error && cpu->hotpluggable) {
>  		register_cpu_control(cpu);
> +		per_cpu(is_hotpluggable, num) = 1;
> +	}

This information already exists in the ->hotpluggable field of "struct
cpu".  I'd suggest an alternate approach (included as a patch below),
which avoids the need to add a new per_cpu property redundant with
->hotpluggable.

Note that you can get to the patch below via "git am --scissors", or
just by copy/pasting it.

---8<---
From: Josh Triplett <josh@joshtriplett.org>
Subject: [PATCH] driver-core/cpu: Expose hotpluggability to the rest of the kernel

When architectures register CPUs, they indicate whether the CPU allows
hotplugging; notably, x86 and ARM don't allow hotplugging CPU 0.
Userspace can easily query the hotpluggability of a CPU via sysfs;
however, the kernel has no convenient way of accessing that property in
an architecture-independent way.  While the kernel can simply try it and
see, some code needs to distinguish between "hotplug failed" and
"hotplug has no hope of working on this CPU"; for example, rcutorture's
CPU hotplug tests want to avoid drowning out real hotplug failures with
expected failures.

Expose this property via a new cpu_is_hotpluggable function, so that the
rest of the kernel can access it in an architecture-independent way.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 drivers/base/cpu.c  |    7 +++++++
 include/linux/cpu.h |    1 +
 2 files changed, 8 insertions(+), 0 deletions(-)

On Sat, Dec 03, 2011 at 01:06:50PM -0800, Josh Triplett wrote:
> On Sat, Dec 03, 2011 at 10:34:41AM -0800, Paul E. McKenney wrote:
> > The rcutorture test now can automatically exercise CPU hotplug and
> > collect success statistics, which can be correlated with other rcutorture
> > activity.  This permits rcutorture to completely exercise RCU regardless
> > of what sort of userspace and filesystem layout is in use.  Unfortunately,
> > rcutorture is happy to attempt to offline CPUs that cannot be offlined,
> > for example, CPU 0 in both the x86 and ARM architectures.  Although this
> > allows rcutorture testing to proceed normally, it confounds attempts at
> > error analysis due to the resulting flood of spurious CPU-hotplug errors.
> > 
> > Therefore, this commit creates a cpu_is_hotpluggable() function that
> > allows rcutorture to avoid attempting to offline CPUs that are not
> > hotpluggable, which in turn allows rcutorture to avoid reporting spurious
> > CPU-hotplug errors.  Note that this function is EXPORT_SYMBOL_GPL()
> > to allow rcutorture to use it when compiled as a kernel module.
> > This commit also includes modifications to rcutorture to use this
> > new function.
> 
> I'd suggest writing this as a two-patch series: add the API, then use
> it.
> 
> I'd also suggest making the more general case for this API, beyond just
> rcutorture.
> 
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -23,6 +23,7 @@ struct sysdev_class cpu_sysdev_class = {
> >  EXPORT_SYMBOL(cpu_sysdev_class);
> >  
> >  static DEFINE_PER_CPU(struct sys_device *, cpu_sys_devices);
> > +static DEFINE_PER_CPU(bool, is_hotpluggable);
> [...]
> > @@ -224,8 +226,10 @@ int __cpuinit register_cpu(struct cpu *cpu, int num)
> >  
> >  	error = sysdev_register(&cpu->sysdev);
> >  
> > -	if (!error && cpu->hotpluggable)
> > +	if (!error && cpu->hotpluggable) {
> >  		register_cpu_control(cpu);
> > +		per_cpu(is_hotpluggable, num) = 1;
> > +	}
> 
> This information already exists in the ->hotpluggable field of "struct
> cpu".  I'd suggest an alternate approach (included as a patch below),
> which avoids the need to add a new per_cpu property redundant with
> ->hotpluggable.

I do like your patch better, as it is smaller, less intrusive, and
makes better use of existing facilities.  I have therefore replaced
mine with it.  I also created a separate patch containing the rcutorture
modifications from my original.

> Note that you can get to the patch below via "git am --scissors", or
> just by copy/pasting it.

Very cool -- wasn't aware of --scissors before.  ;-)

							Thanx, Paul

> ---8<---
> From: Josh Triplett <josh@joshtriplett.org>
> Subject: [PATCH] driver-core/cpu: Expose hotpluggability to the rest of the kernel
> 
> When architectures register CPUs, they indicate whether the CPU allows
> hotplugging; notably, x86 and ARM don't allow hotplugging CPU 0.
> Userspace can easily query the hotpluggability of a CPU via sysfs;
> however, the kernel has no convenient way of accessing that property in
> an architecture-independent way.  While the kernel can simply try it and
> see, some code needs to distinguish between "hotplug failed" and
> "hotplug has no hope of working on this CPU"; for example, rcutorture's
> CPU hotplug tests want to avoid drowning out real hotplug failures with
> expected failures.
> 
> Expose this property via a new cpu_is_hotpluggable function, so that the
> rest of the kernel can access it in an architecture-independent way.
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> ---
>  drivers/base/cpu.c  |    7 +++++++
>  include/linux/cpu.h |    1 +
>  2 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 251acea..3991502 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -247,6 +247,13 @@ struct sys_device *get_cpu_sysdev(unsigned cpu)
>  }
>  EXPORT_SYMBOL_GPL(get_cpu_sysdev);
> 
> +bool cpu_is_hotpluggable(unsigned cpu)
> +{
> +	struct sys_device *dev = get_cpu_sysdev(cpu);
> +	return dev && container_of(dev, struct cpu, sysdev)->hotpluggable;
> +}
> +EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
> +
>  int __init cpu_dev_init(void)
>  {
>  	int err;
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 6cb60fd..305c263 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -27,6 +27,7 @@ struct cpu {
> 
>  extern int register_cpu(struct cpu *cpu, int num);
>  extern struct sys_device *get_cpu_sysdev(unsigned cpu);
> +extern bool cpu_is_hotpluggable(unsigned cpu);
> 
>  extern int cpu_add_sysdev_attr(struct sysdev_attribute *attr);
>  extern void cpu_remove_sysdev_attr(struct sysdev_attribute *attr);
> -- 
> 1.7.7.3
>