linux-kernel - Re: [PATCH 2/3 RFC] Driver core: Use generic offline/online for CPU offline/online

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Date:	Thu, 02 May 2013 02:26:55 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Toshi Kani <toshi.kani@...com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	isimatu.yasuaki@...fujitsu.com,
	vasilis.liaskovitis@...fitbricks.com
Subject: Re: [PATCH 2/3 RFC] Driver core: Use generic offline/online for CPU offline/online

On Wednesday, May 01, 2013 02:07:45 PM Toshi Kani wrote:
> On Wed, 2013-05-01 at 16:49 +0200, Rafael J. Wysocki wrote:
> > On Tuesday, April 30, 2013 05:42:06 PM Toshi Kani wrote:
> > > On Mon, 2013-04-29 at 14:28 +0200, Rafael J. Wysocki wrote:
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > 
> > > > Rework the CPU hotplug code in drivers/base/cpu.c to use the
> > > > generic offline/online support introduced previously instead of
> > > > its own CPU-specific code.
> > > > 
> > > > For this purpose, modify cpu_subsys to provide offline and online
> > > > callbacks for CONFIG_HOTPLUG_CPU set and remove the code handling
> > > > the CPU-specific 'online' sysfs attribute.
> > > > 
> > > > This modification is not supposed to change the user-observable
> > > > behavior of the kernel (i.e. the 'online' attribute will be present
> > > > in exactly the same place in sysfs and should trigger exactly the
> > > > same actions as before).
> > > > 
> > > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > ---
> > > >  drivers/base/cpu.c |   62 ++++++++++++-----------------------------------------
> > > >  1 file changed, 15 insertions(+), 47 deletions(-)
> > > > 
> > > > Index: linux-pm/drivers/base/cpu.c
> > > > ===================================================================
> > > > --- linux-pm.orig/drivers/base/cpu.c
> > > > +++ linux-pm/drivers/base/cpu.c
> > > > @@ -16,66 +16,25 @@
> > > >  
> > > >  #include "base.h"
> > > >  
> > > > -struct bus_type cpu_subsys = {
> > > > -	.name = "cpu",
> > > > -	.dev_name = "cpu",
> > > > -};
> > > > -EXPORT_SYMBOL_GPL(cpu_subsys);
> > > > -
> > > >  static DEFINE_PER_CPU(struct device *, cpu_sys_devices);
> > > >  
> > > >  #ifdef CONFIG_HOTPLUG_CPU
> > > > -static ssize_t show_online(struct device *dev,
> > > > -			   struct device_attribute *attr,
> > > > -			   char *buf)
> > > > +static int cpu_subsys_online(struct device *dev)
> > > >  {
> > > > -	struct cpu *cpu = container_of(dev, struct cpu, dev);
> > > > -
> > > > -	return sprintf(buf, "%u\n", !!cpu_online(cpu->dev.id));
> > > > +	return cpu_up(dev->id);
> > > >  }
> > > >  
> > > > -static ssize_t __ref store_online(struct device *dev,
> > > > -				  struct device_attribute *attr,
> > > > -				  const char *buf, size_t count)
> > > > +static int cpu_subsys_offline(struct device *dev)
> > > >  {
> > > > -	struct cpu *cpu = container_of(dev, struct cpu, dev);
> > > > -	ssize_t ret;
> > > > -
> > > > -	cpu_hotplug_driver_lock();
> > > 
> > > By replacing cpu_hotplug_driver_lock() with lock_device_offline() in
> > > patch 1/3, it no longer protects from other places that still use
> > > cpu_hotplug_device_lock(), such as save_mc_for_early().
> > 
> > Yes.
> > 
> > What about taking cpu_hotplug_driver_lock() around cpu_up() and
> > cpu_down() in cpu_subsys_online() and cpu_subsys_offline()?
> 
> Sounds like a reasonable approach to me. 
> 
> > Alternatively, I can just replace cpu_hotplug_driver_lock() with
> > lock_device_offline() everywhere.
> 
> That works too.  Not sure which way is better.

It turns out that cpu_hotplug_driver_lock() is per-arch, so I'd prefer to
just take it in cpu_subsys_online() and cpu_subsys_offline(), at least for
the time being.

> If we go this option,
> I'd suggest to rename lock_device_offline() since it could be misleading
> that the lock is only used for offline, i.e. excluding online.
> lock_device_hotplug() might be less confusing although we distinguish
> online/offline and hotplug operations.

Well, I've decided to rename them to lock/unlock_device_hotplug() anyway,
because "hotplug" has been used to refer to CPU offline/online for years.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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