Messages in this thread | | | Date | Sat, 8 Oct 2011 16:07:01 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [: [RFC] wake up notifications and suspend blocking (aka more wakelock stuff)] |
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On Sat, 8 Oct 2011, Rafael J. Wysocki wrote:
> On Saturday, October 08, 2011, mark gross wrote: > > On Sat, Oct 08, 2011 at 10:14:39PM +1100, NeilBrown wrote: > > > On Sun, 2 Oct 2011 09:44:56 -0700 mark gross <markgross@thegnar.org> wrote: > > > > > > > resending to wider list for discussion > > > > ----- Forwarded message from mark gross <markgross@thengar.org> ----- > > > > > > > > Subject: [RFC] wake up notifications and suspend blocking (aka more wakelock stuff) > > > > Date: Tue, 20 Sep 2011 13:33:05 -0700 > > > > From: mark gross <markgross@thengar.org> > > > > To: linux-pm@lists.linux-foundation.org > > > > Reply-To: markgross@thegnar.org > > > > Cc: arve@android.com, markgross@thegnar.org, Alan Stern <stern@rowland.harvard.edu>, amit.kucheria@linaro.org, farrowg@sg.ibm.com, "Rafael J. Wysocki" <rjw@sisk.pl> > > > > > > > > The following patch set implement an (untested) solution to the > > > > following problems. > > > > > > > > 1) a method for making a system unable to suspend for critical sections > > > > of time. > > > > > > We already have this. A properly requested suspend (following wakeup_count > > > protocol) is unable to complete between wakeup_source_activate() and > > > wake_source_deactivate() - these delimit the critical sections. > > > > > > What more than this do you need? > > > > sometimes devices that are not wake up sources need critical sections > > where suspend is a problem.
Mark, can you give some examples? This isn't the same as the firmware update thing, is it? That deserves to be handled by a completely separate mechanism.
> > > If user-space wants to prevent suspend, it just needs some sort of protocol > > > for talking to the user-space process which follows the correct protocol to > > > initiate suspend. That isn't a kernel problem. > > > > The devices that I've seen that need to block suspend don't have a > > communication interface to user mode. > > > > But, you are right the devices that need this sort of thing could > > register as wakeup sources and block suspend as well. > > > > FWIW This part of the patch set was to wrap up a proposal I got last > > year from some folks to try to provide somewhat compatible semantics to > > wakelock's for the android and linux kernel community. > > > > I include it for completeness.
Although many of these problems could be solved by adopting a suitable protocol in userspace, currently there is no such protocol. Even if one did exist, the process of getting all the relevant programs to adopt it would take quite a while. This is a case where a problem can be solved either in the kernel or in userspace, and the in-kernel solution may be simpler.
> > > > 2) providing a race free method for the acknowledgment of wake event > > > > processing before re-entry into suspend can happen. > > > > > > Again, this is a user-space problem. It is user-space which requests > > > suspend. It shouldn't request it until it has checked that there are no wake > > > events that need processing - and should use the wakeup_count protocol to > > > avoid races with wakeup events happening after it has checked. > > > > Here you are wrong, or missing the point. The kernel needs to be > > notified from user mode that an update event has been consumed by > > whoever cares about it before the next suspend can happen. > > This, in fact, isn't correct. I have tried to turn your (and John's) > attention to this for quite a few times already. > > The point is that the entity about to trigger suspend (that need not be the > kernel!) has to communicate with the processes that consume wakeup events > beforehand. In theory this communication can happen entirely in user > space, but that would involve quite complicated interactions between > processes, so nobody does that in practice. > > The only "problem" that can't be solved entirely in user space, which is > what John turned my attention to during the LPC, is that it may be > possible to suspend when processes that should be asked about whether or > not to suspend are sleeping and that may be done _without_ actually asking > those processes for permission.
Essentially, there has to be a way for these processes to say "It's okay to suspend until I tell you otherwise." That could also be part of the userspace protocol (if it existed).
> The difficult part is, if we suspend in > such a situation, we need to wait until all of those processes have a chance > to run before attempting to suspend again.
Of course, a power-management daemon could handle that part easily. Doing it in the kernel is more difficult.
> > The fact that there are time outs in the existing wake event code points to > > this shortcoming in the current implementation. > > Actually, the timeouts serve a different purpose. Namely, there are wakeup > events that aren't actually consumed by anyone above the layer signaling the > event (think about Wake-on-LAN via a magic packet) and if such an event > happens, we can't suspend at once, because we need to assume that it happened > for a reason, so whoever triggered the event has to be given a chance to do > whatever he needed to wake up the system for. This cannot be achieved without > timeouts.
Also there are events that may or may not get consumed, but it is impossible to tell at the time whether or not a consumer exists. If one does exist then good, the system can suspend again when the consumer is finished. But if not, the system can only wait until a timeout.
> > I suppose one could rig up the user mode suspend daemon with > > notification callbacks between event consumers across the user mode > > stack but its really complex to get it right and forces a solution to a > > problem better solved in kernel mode be done with hacky user mode > > gyrations that may ripple wildly across user mode. > > Agreed. > > > Also it is the kernel that is currently deciding when to unblock the > > suspend daemon for the next suspend attempt. Why not build on that and > > make is so we don't need the time outs? > > > > > i.e. there is no kernel-space problem to solve here (except for possible > > > bugs). > > > > Just a race between the kernel allowing a suspend and the user mode code > > having time to consume the last wake event. > > That's correct.
Alan Stern
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