On Mon, 21 Jun 2010, Florian Mickler wrote: > On Sun, 20 Jun 2010 22:23:38 -0400 (EDT) > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > This is the race I was talking about: > > > > > > What happens if an event arrives just before you read > > > > /sys/power/wakeup_count, but the userspace consumer doesn't realize > > > > there is a new unprocessed event until after the power manager checks > > > > it? > > > > > I think this is not the kernel's problem. In this approach the kernel makes it > > > possible for the user space to avoid the race. Whether or not the user space > > > will use this opportunity is a different matter. > > > > It is _not_ possible for userspace to avoid this race. Help from the > > kernel is needed. > > It is possible if every (relevant) userspace program implements a > callback for the powermanager to check if one of it's wakeup-sources > got activated. > > That way the powermanager would read /sys/power/wakeup_count, then do > the roundtrip to all it's registered users and only then suspend. > > This turns the suspend_blockers concept around. Instead of actively > signaling the suspend_blockers, the userspace programs only answer > "yes/no" when asked. (i.e. polling?) In the end you would want to have communication in both directions: suspend blockers _and_ callbacks. Polling is bad if done too often. But I think the idea is a good one. In fact, you don't need a "yes/no" response. Programs merely need a chance to activate a new suspend blocker if a wakeup source was recently activated before they acknowledge the poll. > You _can not_ implement userspace suspend blockers with this approach, > as it is vital for every userspace program to get scheduled and check > it's wakeup-source (if even possible) before you know that the right > parties have won the race. I'm not sure what you mean. Certainly you can take a userspace suspend-blocker implementation of the sort discussed before (where programs communicate their needs to a central power-manager process) and add this callback mechanism on top. There is still at least one loophole to be closed: Android's timer-based wakelocks. These include cases where the Android developers didn't add enough wakelocks to cover the entire path from kernel-event to userspace-handler, so they punted and relied on a timer to decide when the wakelock should be deactivated. (There may be other cases too; I didn't follow the original discussion very closely.) It's not clear whether these things can be handled already in Rafael's scheme with your addition, or whether something new is needed. Alan Stern _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
