question about process freezing

question about process freezing

[Bob YANG]

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Hi,

Could anybody explain to me what process freezing is? I mean, what will
happen to a frozen process? I tried to understand the codes in
kernel/power/process.c and found one thing freeze_process() does is to set
PF_FREEZE flag. What is that for?

Thanks,
Peng

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Re: question about process freezing

[Rafael J. Wysocki]

Hi,

On Monday, 11 December 2006 03:05, Bob YANG wrote:
> Hi,
> 
> Could anybody explain to me what process freezing is? I mean, what will
> happen to a frozen process? I tried to understand the codes in
> kernel/power/process.c and found one thing freeze_process() does is to set
> PF_FREEZE flag. What is that for?

This is to tell the process to go to the refrigerator.

The function used by processes to check if we want them to go to the
refrigerator is called try_to_freeze() and defined in either sched.h or
freezer.h (in newer kernels).  User space processes call try_to_freeze()
in get_signal_to_deliver() and kernel threads (that are freezeable) should
call it explicitly wherever appropriate.

The refrigerator itself is a function defined in process.c.  Processes that
enter it set their PF_FROZEN flags and change their states to
TASK_UNINTERRUPTIBLE.   They are in this state until we clear the PF_FROZEN
flag for them, in thaw_processes().

Greetings,
Rafael


-- 
If you don't have the time to read,
you don't have the time or the tools to write.
		- Stephen King

Question about process freezing

[Franck Bui-Huu]

Hi,

I have a simple/dumb question about process freezing.

Instead of freezing every single user land processes by sending them a
fake signal and waiting for them to go to the refrigerator, why not
freezing the scheduler ?

Thanks,
		Franck

Re: Question about process freezing

[Alan Stern]

On Tue, 20 Nov 2007, Franck Bui-Huu wrote:

> Hi,
> 
> I have a simple/dumb question about process freezing.
> 
> Instead of freezing every single user land processes by sending them a
> fake signal and waiting for them to go to the refrigerator, why not
> freezing the scheduler ?

Because processes can't simply be frozen immediately.  They need to 
have a chance to release any important locks or mutexes they may be 
holding.

Alan Stern

Re: Question about process freezing

[Franck Bui-Huu]

Alan Stern wrote:
> On Tue, 20 Nov 2007, Franck Bui-Huu wrote:
>> Instead of freezing every single user land processes by sending them a
>> fake signal and waiting for them to go to the refrigerator, why not
>> freezing the scheduler ?
> 
> Because processes can't simply be frozen immediately.  They need to 
> have a chance to release any important locks or mutexes they may be 
> holding.
> 

You mean any locks held by a frozen process and needed by the suspend
code path ?

thanks,
		Franck

Re: Question about process freezing

[Rafael J. Wysocki]

On Wednesday, 21 of November 2007, Franck Bui-Huu wrote:
> Alan Stern wrote:
> > On Tue, 20 Nov 2007, Franck Bui-Huu wrote:
> >> Instead of freezing every single user land processes by sending them a
> >> fake signal and waiting for them to go to the refrigerator, why not
> >> freezing the scheduler ?
> > 
> > Because processes can't simply be frozen immediately.  They need to 
> > have a chance to release any important locks or mutexes they may be 
> > holding.
> > 
> 
> You mean any locks held by a frozen process and needed by the suspend
> code path ?

Yes.

Regards,
Rafael