Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: CLOCK_MONOTONIC after suspend | Date | Wed, 15 Dec 2021 20:17:58 +0100 |
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Dirk,
On Wed, Dec 15 2021 at 18:30, info@engel-internet.de wrote: > dT = (T_KS_asleep – T_US_asleep) + (T_US_awake – T_KS_awake) // T: point > in time, KS: kernel space, US: user space > > With a simple user space program that prints out the monotonic time each > 100ms along with the day time, I did some measurements on my notebook. > It reveals the following discrepancies (time gaps) between the last time > stamp written before suspend and the first time stamp after resume: > > dT in [s] #1 #2 #3 #4 #5 #6 #7 > > Suspend2RAM 6.409 6.423 7.451 3.444 7.815 5.655 7.178 > > Suspend2Disk 5.228 2.683 5.072 5.198 4.806 5.763 6.908 > > Is this effect known and accepted or is there some way to prevent or > mitigate it?
there is not much the kernel can do about that.
Timekeeping can only stop at the very latest moment and has to resume immediately when the CPU comes back. That's a matter of internal correctness.
Yes, user space has to be frozen first in order to make that work and is obviously unfrozen last. So the timeline looks like this:
T0 suspend is initiated T1 user space freeze T2 kernel shuts down - timekeeping freeze T3 kernel resumes - timekeeping resume T4 user space unfreeze
So the deltas T2 - T1, T4 - T3 are what matter for your user space program. Those deltas heavily depend on the amount of drivers, outstanding disk operations etc. So your milage will vary.
Thanks,
tglx
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