linux-kernel - Re: CLOCK_MONOTONIC after suspend

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Message-ID: <87o85hr82h.ffs@tglx>
Date:   Wed, 15 Dec 2021 20:17:58 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     info@...el-internet.de, linux-kernel@...r.kernel.org
Subject: Re: CLOCK_MONOTONIC after suspend

Dirk,

On Wed, Dec 15 2021 at 18:30, info@...el-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