Re: Suspend-to-ram/disk signal

[Henrique de Moraes Holschuh <hmh@hmh.eng.br>]

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...

> 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.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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