Re: [RFC][PATCH 0/3] PM: Asynchronous suspend and resume

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On Fri, 2009-08-14 at 02:08 +0800, Rafael J. Wysocki wrote:> On Thursday 13 August 2009, Zhang Rui wrote:> > On Thu, 2009-08-13 at 05:43 +0800, Rafael J. Wysocki wrote:> > > On Wednesday 12 August 2009, Alan Stern wrote:> > > > On Wed, 12 Aug 2009, Rafael J. Wysocki wrote:> > > > > > > > > Hi,> > > > > > > > > > The following patches introduce a mechanism allowing us to execute device> > > > > drivers' suspend and resume callbacks asynchronously during system sleep> > > > > transitions, such as suspend to RAM.  The idea is explained in the [1/1] patch> > > > > message.> > > > > > > > > > Comments welcome.> > > > > > > > I get the idea.  Not bad.> > > > > > Thanks!> > > > > > > Have you tried it in a serious way?  For example, turning on the> > > > async_suspend flag for every device?> > > > > > No, I've only tested it with a few selected drivers.  I'm going to try the> > > "async everyone" scenario, though.> > > > > > > In one way it isn't as efficient as it could be.  You fire off a bunch> > > > of async threads and then make many of them wait for parent or child> > > > devices.  They could be doing useful work instead.> > > > > are you talking about this scenario, or I find another problem of this> > approach:> > there is a part of dpm_list, dev1->dev_aaa->...->dev_bbb->dev2> > > > dev2 is dev1's first child.> > dev1 resume takes 1s> > dev_aaa~dev_bbb resume takes 0.1s.> > > > if we call device_enable_async_suspend(dev1, true) in order to resume> > device1 asynchronously, the real asynchronous resume only happens> > between dev1 and dev_aaa to dev_bbb because dev2 needs to wait until> > dev1 resume finished.> > Yes, that's how it works, but I would call it a limitation rather than a> problem.  It partially stems from the fact that __async_schedule() executes> ptr() synchronously in some circumstances (e.g. async_enabled unset), so the> suspend and resume callbacks have to be scheduled in the same order, in which> they would have been called synchronously.> > > so kernel schedules dev1 resume in an async thread first, and then takes> > 0.1s to finish the dev_aaa to dev_bbb resume, and then sleep 0.9s> > > > > I kind of agree, but then the patches would be more complicated.> > > > > The problem is that we need to invoke device_resume for every device> > synchronously.> > Yes, we do.> > > I wonder if we can make the child devices inherit the> > parent's dev->power.async_suspend flag, so that devices that need to> > wait are resumed asynchronously, i.e. we never wait/sleep when parsing> > the dpm_list.> > > > this doesn't bring too much benefit in suspend case but it can speed up> > the resume process a lot.> > We can do that at the core level,
I think you mean we can't do that at the core level.
>  because there may be dependencies between> the children the core doesn't know about.  Subsystems are free to set> async_suspend for the entire branches of device hierarchy if they are known> not to contain any off-tree dependencies, but the core has no information> about that.> hmm, but the current patch doesn't handle the off-tree dependenciesneither.e.g.dev1, dev2, dev3dev2 depends on dev1, dev3 is dev1's first child,we only promise that dev1 has been resumed before resuming dev3 in thecurrent proposal.
anyway, this is not a problem after the pm_connection stuff isimplemented. :)
thanks,rui> > Of cause, this is not a problem if we turn on the async_suspend flag for> > every device.> > Yes, but we cannot do that at this point.> > > > > It would be interesting to invent a way of representing explicitly the > > > > non-tree dependencies -- assuming there aren't too many of them!  (I > > > > can just hear the TI guys hollering about power and timer domains...)> > > > > > I have an idea.> > > > > > Every such dependency involves two devices, one of which is a "master"> > > and the second of which is a "slave", meaning that the "slave" have to be> > > suspended before the "master" and cannot be resumed before it.  In principle> > > we could give each device two lists of "dependency objects", one containing> > > "dependency objects" where the device is the "master" and the other containing> > > "dependency objects" where the device is the "slave".  Then, each "dependency> > > object" could be represented as> > > > > > struct pm_connection {> > >     struct device *master;> > >     struct list_head master_hook;> > >     struct device *slave;> > >     struct list_head slave_hook;> > > };> > > > > > Add some locking, helpers for adding / removing "dependency objects" etc.> > > and it should work.  Instead of checking the parent, walk the list of> > > "masters", instead of walking the list of children, walk the list of "slaves".> > > > > > The core could create those objects for parent-child relationships> > > automatically, the other ones would have to be added by platforms / bus types /> > > drivers etc.> > > > > this sounds great. :)> > It can only be the next step, though, because it will affect the runtime PM as> well,  among other things.> 


> Thanks,> Rafael
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