Re: Suspend-to-ram/disk signal

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

On Wednesday, 28 of November 2007, Henrique de Moraes Holschuh wrote:
> On Tue, 27 Nov 2007, Rafael J. Wysocki wrote:
> > On Monday, 26 of November 2007, Pascal d'Hermilly wrote:
> > > When the computer suspends and comes back does it send a signal to the 
> > > running applications?
> > 
> > No, it doesn't.  Applications aren't supposed to notice the suspend.
> 
> Except, of course, when they need to notice the suspend.  The clocks
> changed, the hardware configuration may have changed, and something
> certainly did happen, so "Applications aren't supposed to notice the
> suspend" is a naïve proposition at best.  They need to *deal well* with it,
> instead.
> 
> ntp needs to reset everything, for example.  A LOT of software really
> dislikes large steps in the clocks (monotonic or gettimeofday()), and
> misbehave.  Not notifying them they need to resync themselves is part of the
> breakage.  Multimedia applications and screen savers are often plagued by
> this sort of problem.
> 
> A deskptop suite trying to emulate the ThinkVantage suite in a ThinkPad
> might want to check that the user woke up the laptop just to eject the bay,
> and ask him if he wants the box to go back to sleep, and maybe even offer to
> do so automatically from now on.
> 
> The possibilities are endless...

OTOH, there are applications that need not be notified in any way, so I don't
think we should uncondtionally send signals to all of them, for example.

> > Your distribution surely uses some scripts that activate the kernel's suspend
> > code.  You can modify these scripts to notify your application.
> 
> Which is, of course, one way to work around the issue (and probably the best
> one, since trying to act upon a wakeup out-of-sync with whatever said script
> might be doing is likely not going to be very wise).
> 
> But it would still be nice if we kicked userspace in the arse to let it know
> it was sleeping for a while and needs to resync, when we wake up each
> userspace task.   The kernel makes that information available to kthreads
> for a damn good reason.  Sounds like a job for a SIGCONT, but I don't know
> how well would that work.

It wouldn't work very well, IMHO, unless the tasks in question are prepared to
get an "out of order" SIGCONT.

However, sending SIGSTOP to selected tasks before the suspend and then SIGCONT
after the resume might do the trick.

Greetings,
Rafael
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