Date:	Fri, 28 Oct 2011 13:57:01 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	a.p.zijlstra@...llo.nl, stern@...land.harvard.edu, pavel@....cz,
	len.brown@...el.com, mingo@...e.hu, akpm@...ux-foundation.org,
	suresh.b.siddha@...el.com, lucas.demarchi@...fusion.mobi,
	linux-pm@...r.kernel.org, rusty@...tcorp.com.au,
	vatsa@...ux.vnet.ibm.com, ashok.raj@...el.com,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	rdunlap@...otime.net, Tejun Heo <tj@...nel.org>,
	Borislav Petkov <bp@...64.org>
Subject: Re: [PATCH v4 2/2] CPU hotplug, Freezer: Synchronize CPU hotplug and Freezer

On Friday, October 28, 2011, Srivatsa S. Bhat wrote:
> On 10/28/2011 01:43 AM, Rafael J. Wysocki wrote:
> > Hi,
> > 
> > On Thursday, October 27, 2011, Srivatsa S. Bhat wrote:
> >> Prevent CPU hotplug and the freezer from racing with each other, to ensure
> >> that during the *entire duration* for which the callbacks for CPU hotplug
> >> notifications such as CPU_ONLINE[_FROZEN], CPU_DEAD[_FROZEN] etc are being
> >> executed, the state of the system (with respect to the tasks being frozen
> >> or not) remains constant.
> >>
> >> This patches hooks the CPU hotplug infrastructure onto the freezer
> >> notifications (PM_FREEZE_PREPARE and PM_POST_THAW) and thus synchronizes
> >> with the freezer.
> >>
> >> Specifically,
> >>
> >> * Upon the PM_FREEZE_PREPARE notification, the CPU hotplug callback disables
> >>   future (regular) CPU hotplugging and also ensures that any currently running
> >>   CPU hotplug operation is completed before allowing the freezer to continue
> >>   any further.
> >>
> >> * Upon the PM_POST_THAW notification, the CPU hotplug callback re-enables
> >>   regular CPU hotplug.
> >>
> >> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> >> ---
> >>
> >>  kernel/cpu.c |   76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 files changed, 76 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/kernel/cpu.c b/kernel/cpu.c
> >> index 12b7458..61985ce 100644
> >> --- a/kernel/cpu.c
> >> +++ b/kernel/cpu.c
> >> @@ -15,6 +15,7 @@
> >>  #include <linux/stop_machine.h>
> >>  #include <linux/mutex.h>
> >>  #include <linux/gfp.h>
> >> +#include <linux/suspend.h>
> >>  
> >>  #ifdef CONFIG_SMP
> >>  /* Serializes the updates to cpu_online_mask, cpu_present_mask */
> >> @@ -478,6 +479,81 @@ static int alloc_frozen_cpus(void)
> >>  core_initcall(alloc_frozen_cpus);
> >>  #endif /* CONFIG_PM_SLEEP_SMP */
> >>  
> >> +
> >> +#ifdef CONFIG_FREEZER
> >> +
> >> +/*
> >> + * Avoid CPU hotplug racing with the freezer subsystem, by disabling CPU
> >> + * hotplug when tasks are about to be frozen.
> >> + *
> >> + * Also, don't allow the freezer subsystem 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 is completed.
> >> + */
> >> +static void cpu_hotplug_freezer_block_begin(void)
> >> +{
> >> +	cpu_maps_update_begin();
> >> +	cpu_hotplug_disabled = 1;
> >> +	cpu_maps_update_done();
> >> +}
> >> +
> >> +
> >> +/*
> >> + * When thawing of tasks is complete, re-enable CPU hotplug (which had been
> >> + * disabled while beginning to freeze tasks).
> >> + */
> >> +static void cpu_hotplug_freezer_block_done(void)
> >> +{
> >> +	cpu_maps_update_begin();
> >> +	cpu_hotplug_disabled = 0;
> >> +	cpu_maps_update_done();
> >> +}
> >> +
> > 
> > I wonder if the new PM notifier events are really necessary?
> > 
> > Why don't you just call cpu_hotplug_freezer_block_begin() (perhaps
> > with a better name?) directly from freeze_processes()?  And analogously
> > for cpu_hotplug_freezer_block_done() and thaw_processes()?
> > 
> 
> Yes, we can definitely do that. 
> 
> But the reason why I chose to introduce new notifiers was to make this
> more extensible (because we know that at least 2 subsystems would benefit
> from mutually excluding themselves from the freezer, namely CPU hotplug
> and x86 microcode).
> http://thread.gmane.org/gmane.linux.kernel/1198291/focus=1200591
> 
> But now that I think of it, hooking onto the freezer notifiers wouldn't
> solve the microcode cases since usermodehelper_disable() is called
> _before_ freezing tasks... :(
> 
> So we should probably call the functions directly like you suggested.. 
> 
> But I really didn't want to clutter the freezer call path because of problems
> elsewhere. So I felt freezer notifiers would be a cleaner way of dealing with
> such things. Also, since freezer is a generic subsystem that could be used
> for purposes other than S3/S4 as well (I have heard of attempts to use freezer
> during tracing), wouldn't it be better to introduce new notifiers to
> announce the begin and end of freezer activity to interested subsystems?
> (and then use them to solve the CPU hotplug issue like this patch does...)
> 
> Please let me know your suggestions.

The freeze_processes() and thaw_processes() functions are only used for
system suspend and hibernation, as far as I can tell, and I don't think there
will be any other users in predictable future.

Also, adding the calls directly to those functions will show exactly what
the dependecies are, while doing that through a notifier kind of obfuscates
things.  So, please make direct calls from there.

Thanks,
Rafael
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