Re: shrinking memory for suspend?

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, 30 April 2007 17:09, Johannes Berg wrote:
> Hi,
> 
> > I don't think it's related to the freezing of user space.  You can allocate
> > memory with all of the user space frozen just fine.  I'd say it's related to
> > the freezing of kernel threads, rather.
> 
> Ah, true, of course.
> 
> > > Why do we freeze userspace anyway?
> > 
> > Well, let me quote Linus:
> > 
> > "I _do_ realize the IO request queue issues, and that we cannot actually do 
> > s2ram with some devices in the middle of a DMA. So we want to be able to 
> > avoid *that*, there's no question about that. And I suspect that stopping 
> > user threads and then waiting for a sync is practically one of the easier 
> > ways to do so."
> 
> Hmm, yeah. Does this apply to kernel threads too? Actually, I guess
> device drivers should wait for suspend and reject commands to devices
> that are suspended already?

Kernel threads that belong to device drivers are frozen too (well, not exactly
all of them, but let's omit the ugly details for now), because they generally
need to be synchronized with the drivers' .suspend() and .resume() routines
and IMO it's easier to freeze them than handle that in any other way.

After .suspend() and before .resume() any commands should not be sent to the
device.

> Can it actually happen that we schedule to userspace while calling all the
> suspend issues etc? 

No, the user space is frozen (ie. in TASK_UNINTERRUPTIBLE) at that time.

Greetings,
Rafael