Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2:hang in atomic copy)

[Nigel Cunningham]

Hi.

On Wed, 2007-04-25 at 19:04 -0700, Linus Torvalds wrote:
> 
> On Thu, 26 Apr 2007, Nigel Cunningham wrote:
> >
> > That's where I think you're overstretching the argument. Like suspend 
> >(to ram), we're concerned at the snapshot point with getting the hardware 
> >in the same state at a later stage.
> 
> Really, no.
> 
> "suspend to ram" doesn't _have_ a "snapshot point".

Sorry. I wasn't clear. I wasn't saying that suspend to ram has a
snapshot point. I was trying to say it has a point where you're seeking
to save information (PCI state / SCSI transaction number or whatever)
that you'll need to get the hardware into the same state at a later
stage. That (saving information) is the point of similarity.

> I've tried to explain this multiple times, I don't know why it's not 
> apparently sinking in. This is much more fundamental than the fact that 
> you don't want to stop disks for snapshotting, although it really boils 
> down to all the same issues: the operations are simply not at all the 
> same!

Miscommunication, I think. Does the above help?

> I agree 100% that "snapshot to disk" is a "snapshot event". You have to 
> create a single point in time when everything is stable. And I'd much 
> rather call it "snapshot to disk" than "suspend to disk" to make it clear 
> that it's something _totally_ different from "suspend".
> 
> Because the thing is, "suspend to ram" is *not* a snapshot event. At no 
> point do you actually need to "snapshot" the system at all. You can just 
> gradually shut more and more things down, and equally gradually bring them 
> back up. There simply is *never* any "snapshot" time from a device 
> standpoint, because you can just shut down devices in the right order AND 
> YOU ARE DONE.
> 
> Really. 

I suppose that's another point of similarity - for snapshotting, the
same ordering is probably needed?

> [ Obviously s2ram does have one "magic moment", namely the time when the 
>   CPU does the magic read from the northbridge that actually turns off 
>   power for the CPU. But that's really a total non-event from a device 
>   standpoint, so while it's undoubtedly a very interesting moment in the 
>   suspend sequence, it's not really relevant in any way for device 
>   drivers in general. Not at all like the "snapshot moment" that requires 
>   the whole system to be totally quiescent in a "snapshot to disk"! ]
> 
> And the reason s2ram doesn't have a that "snapshot" moment is exactly that 
> the RAM contents are just always there, so there's no need to have a 
> "synchronization event" when ram and devices match. The RAM will *always* 
> match whatever any particular device has done to it, and the proper way to 
> handle things is to just do a simple per-device "save-and-suspend" event.

Yeah.

> And yes, the _individual_ "save-and-suspend" events obviously needs to be 
> "atomic", but it's purely about that particular individual device, so 
> there's never any cross-device issues about that.

No interdependencies? I'm not sure.

> For example, if you're a USB hub controller, which is just about the most 
> complex issue you can have, you obviously want to "save the state" with 
> the controller in a STOPPED state, but that should just go without saying: 
> if the controller isn't stopped, you simply *cannot* save the state, since 
> the state is changing under you. 
> 
> The difference is, that the USB driver needs to just "stop, save, and 
> suspend" as one simple operation for s2ram. In contrast, when doing 
> snapshot to disk, it cannot do that, because while it does want to do the 
> "stop" part, it needs to do so _separately_ from the "save" part because 
> you need to stop everything else *too* before you "save" anythng at all.

Agree.

Nigel

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