Re: [PATCH V2 0/4] introduce device async actions mechanism

[Rafael J. Wysocki]

On Tuesday 11 August 2009, Alan Stern wrote:
> On Tue, 11 Aug 2009, Rafael J. Wysocki wrote:
> 
> > > The general algorithm for maximum parallelism goes as follows: Start by
> > > resuming (in parallel) all the devices which don't depend on anything
> > > else.  Each time a resume finishes, you go on to resume (in parallel)
> > > all the devices which depend only on resumed devices and which haven't
> > > yet started to resume.
> > > 
> > > As described, this can require a large number of threads.  It also
> > > requires detailed knowledge of which devices depend on others, which we
> > > don't have.
> > 
> > It's even more complicated than that.
> > 
> > Assume we have 7 devices, A-G, such that A is the parent of B and C,
> > B is the parent of D and E, and C is the parent of F and G.  Assume in addition
> > that the PM dependencies between the devices are fully reflected by the
> > device tree structure (ie. there are no dependencies that aren't reflected
> > parent-child relationships) and that B and G take 0.5 s to resume while the
> > others take < 1 ms each.  So, the total sequential resume time is
> > 2 s + O(1 ms).
> > 
> > Now, if we used the above algorithm, we'd first resume DEFG which would take
> > 1 s because of G, then we'd resume BC which would take 1 s because of B and
> > the total resume time is again 2 s + O(1 ms).
> 
> (You probably mean "suspend" instead of "resume".)

Yes, I got it the other way around here, sorry.

> No, you misunderstood my description.  We'd start out by suspending
> DEFG.  DEF will finish quickly, at which time we would start suspending 
> B because all its dependencies are satisfied.  When G finishes we would 
> start suspending C.  When B and C are both finished, we would suspend 
> A.  Total time would be about 1 s because B would be started shortly 
> after G.

Ah, OK.

> > However, one can observe that B doesn't need to wait for G to resume, because
> > they are independent of each other.  So, we can resume BDE in parallel with
> > CFG, while of course DE have to wait for B and so on, but this way we can
> > theoretically reduce the total resume time to 1 s + O(1 ms).
> > 
> > The question is how to do that and it seems to me that we can use completions
> > for this purpose.  Namely, add a completion to each device with the following
> > rules:
> > 1) all completions are reset before dpm_resume(),
> > 2) before executing the ->resume() callback for device D, we wait for the
> >    completion of the D's parent,
> > 3) we complete the D's completion after executing its ->resume() callback.
> > Also, the items executed in parallel are now the "wait for the parent's
> > completion, run our callback and complete our completion" things.
> 
> Yes, that's essentially what I described.

Good.

> > At first sight I don't see anything fundamentally wrong with this approach.
> 
> Nothing fundamentally wrong.  The problems come in the details.  Most
> notably, the dependencies that are not reflected in the tree structure.
> 
> In practice, rather than completions I'd recommend using a pool of
> threads together with a single wait queue and a list of devices which
> _might_ be ready.  Initially this list can contain every device.
> 
> Each time a thread wakes up it scans the list, removing devices that
> aren't actually ready yet (i.e., the dependencies aren't satisfied).  
> If it doesn't find any devices that are ready, it goes back to sleep.  
> When it finds a device that _is_ ready, it wakes up another thread and
> invokes the callback.  When the callback is done, the thread adds to
> the list all devices that depend on the one just finished.  Then it
> goes back to scanning the list.

That looks quite complicated at first sight, but asynchronous resume with
completions can be done in a relatively simple patch.  I've got one, so
tomorow I'll post it for further discussion.

Thanks,
Rafael


PS
Did you have a chance to look at rev. 15 of the runtime PM patch
(http://patchwork.kernel.org/patch/40306/)?

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