On Wed, Dec 01, 2010 at 10:43:59AM +0000, Jamie Lokier wrote: > Lennart Poettering wrote: > > On Tue, 23.11.10 19:22, Alexander Shishkin (virtuoso@xxxxxxxxx) wrote: > > > > > Certain userspace applications (like "clock" desktop applets or cron or > > > systemd) might want to be notified when some other application changes > > > the system time. There are several known to me reasons for this: > > > - avoiding periodic wakeups to poll time changes; > > > - rearming CLOCK_REALTIME timers when said changes happen; > > > - changing system timekeeping policy for system-wide time management > > > programs; > > > - keeping guest applications/operating systems running in emulators > > > up to date. > > > > > > This is another attempt to approach notifying userspace about system > > > clock changes. The other one is using an eventfd and a syscall [1]. In > > > the course of discussing the necessity of a syscall for this kind of > > > notifications, it was suggested that this functionality can be achieved > > > via timers [2] (and timerfd in particular [3]). This idea got quite > > > some support [4], [5], [6] and some vague criticism [7], so I decided > > > to try and go a bit further with it. > > > > I agree with Kay, this is pretty much exactly what we want for > > systemd. (Assuming that the time jump due to system suspend is > > propagated to userspace like any other time jump with this path). > > I hope the time jump due to suspend is *not* propagated in the same > way to userspace :-) Can you please check if my new patch from earlier today [1] seems better? [1] http://marc.info/?l=linux-kernel&m=129116762414291&w=2 > What I'd like to see: > > 1. Time jump due to the system clock being stepped: Notification. > > This is *not* a change in real time. It means the clock was > corrected/changed. No physical time passed. > > 2. Time jump due to suspend/resume: Different notification. > > This *is* a change in real time. Physical time passed. > > 3. Time drift corrections: As now, no notification, it's just > the clock being regulated. Generally, I used CLOCK_MONOTONIC + TFD_NOTIFY_CLOCK_SET to signal suspend/resume time changes and CLOCK_REALTIME + TFD_NOTIFY_CLOCK_SET to signal wall clock changes. Without TFD_NOTIFY_CLOCK_SET it works as it did before. > To signal the difference between 1 and 2, there ought to be some way > for userspace to determine how much of the clock delta corresponds > with physical time, by reading some sort of "monotonic" clock :-) > > CLOCK_MONOTONIC is unsuitable because it stops at suspend. Maybe it > should stay that way. But maybe not - programs using CLOCK_MONOTONIC > usually want to trigger timeouts etc. based on real elapsed time, and > after suspend/resume, it's quite reasonable to want to trigger all of > a program's short timeouts immediately. Indeed some network protocol > userspace may currently behave *incorrectly* over suspend/resume, > especially those using clock times to validate their caches, > *because* CLOCK_MONOTONIC doesn't count it. > > So maybe CLOCK_MONOTONIC should be changed to include elapsed time > during suspend/resume, and CLOCK_MONOTONIC_RAW could remain as it is, > for programs that want that? > > That, plus this proposed patch, would signal the difference between 1 > and 2 above nicely. Regards, -- Alex -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
