Re: [GIT PULL] PM updates for 2.6.33

[Rafael J. Wysocki]

On Sunday 06 December 2009, Linus Torvalds wrote:
> 
> On Sun, 6 Dec 2009, Rafael J. Wysocki wrote:
> > 
> > The approach you're suggesting would require modifying individual drivers which
> > I just wanted to avoid.
> 
> In the init path, we had the reverse worry - not wanting to make 
> everything (where "everything" can be some subsystem like just the set of 
> PCI drivers, of course - not really "everything" in an absolute sense) 
> async, and then having to try to work out with the random driver that 
> couldn't handle it.
> 
> And there were _lots_ of drivers that couldn't handle it, because they 
> knew they got woken up serially. The ATA layer needed to know about 
> asynchronous things, because sometimes those independent devices aren't so 
> independent at all. Which is why I don't think your approach is safe.

While the current settings are probably unsafe (like enabling PCI devices
to be suspended asynchronously by default if there are not any direct
dependences between them), there are provisions to make eveything safe, if
we have enough information (which also is needed to put the required logic into
the drivers).  The device tree represents a good deal of the dependences
between devices and the other dependences may be represented as PM links
enforcing specific ordering of the PM callbacks.

> Just to take an example of the whole "independent devices are not 
> necessarily independent" thing - things like multi-port PCMCIA controllers 
> generally show up as multiple PCI devices. But they are _not_ independent, 
> and they actually share some registers. Resuming them asynchronously might 
> well be ok, but maybe it's not. Who knows?

I'd say if there's a worry that the same register may be accessed concurrently
from two different code paths, there should be some locking in place.

> In contrast, a device driver can generally know that certain _parts_ of 
> the initialization is safe. As an example of that, I think the libata 
> layer does all the port enumeration synchronously, but then once the ports 
> have been identified, it does the rest async. 
> 
> That's the kind of decision we can sanely make when we do the async part 
> as a "drivers may choose to do certain parts asynchronously". Doing it at 
> a higher level sounds like a problem to me.

The difference between suspend and initialization is that during suspend we
have already enumerated all devices and we should know how they depend on
each other (and we really should know that if we are to actually understand how
things work), so we can represent that information somehow and use it to do
things at the higher level.

How to represent it is a different matter, but in principle it should be
possible.

> > If you don't like that, we'll have to take the longer route, although 
> > I'm afraid that will take lots of time and we won't be able to exploit 
> > the entire possible parallelism this way.
> 
> Sure. But I'd rather do the safe thing. Especially since there are likely 
> just a few cases that really take a long time.

And there are lots of small sleeps here and there that accumulate and are
entirely avoidable.

> > During suspend we actually know what the dependences between the devicces
> > are and we can use that information to do more things in parallel.  For
> > instance, in the majority of cases (I'm yet to find a counter example), the
> > entire suspend callbacks of "leaf" PCI devices may be run in parallel with each
> > other.
> 
> See above. That's simply not at all guaranteed to be true. 
> 
> And when it isn't true (ie different PCI leaf devices end up having subtle 
> dependencies), now you need to start doing hacky things. 
> 
> I'd much rather have the individual drivers say "I can do this part in 
> parallel", and not force it on them. Because it is definitely _not_ 
> guaranteed that PCI devices can do parallel resume and suspend.

OK, it's not guaranteed, but why not to do this on systems where it's known
to work?

> > Yes, we can do that, but I'm afraid that the majority of drivers won't use the
> > new hooks (people generally seem to be to reluctant to modify their
> > suspend/resume callbacks not to break things).
> 
> See above - I don't think this is a "majority" issue. I think it's a 
> "let's figure out the problem spots, and fix _those_". IOW, get 2% of the 
> coverage, and get 95% of the advantage.

I wouldn't really like to add even more suspend/resume callbacks for this
purpose, because we already have so many of them.  And even if we do that,
I don't really expect drivers to start using them any time soon.

> > Disk spinup/spindown takes time, but also some ACPI devices resume slowly,
> 
> We actually saw that when we did async init. And it was horrible. There's 
> nothing that says that the ACPI stuff necessarily even _can_ run in 
> parallel. 
> 
> I think we currently only do the ACPI battery ops asynchronously.

There are only a few ACPI devices that have real suspend/resume callbacks
and I haven't see problems with these in practice.

Thanks,
Rafael
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