[UM] Long loop in __getnsdayoftime() after resume from ram

[UM] Long loop in __getnsdayoftime() after resume from ram

[Thomas Meyer]

Hello,

in UML kernel I get a long cpu using loop in __getnstimeofday()
(kernel/time/timekeeping.c:315) in the call of timespec_add_ns(),
when I left the host kernel suspended to ram for a few hours and resume
again.
this is because it seems like the tk->xtime_sec wasn't updated yet, but
the nsecs were. nsecs can be as high as 8111000111000111000l

the function timespec_add_ns() (include/linux/time.h:266) will call
__iter_div_u64_rem() which has an optimized loop for the case
that the dividend is not much bigger as the divisior.
but this isn't the case for resume from ram on the host kernel.

any ideas how to fix this? is it possible to intercept the resume from
ram and update the timekeeper->xtime_sec somehow?
or can the um arch somehow overwrite timespec_add_ns() to always use
div_u64_rem() instead?

with kind regards

PS: repost on these lists, because nobody did respond to my original
email.

Re: [uml-devel] [UM] Long loop in __getnsdayoftime() after resume from ram

[Richard Weinberger]

On Sun, Oct 19, 2014 at 2:39 PM, Thomas Meyer <thomas@m3y3r.de> wrote:
> Hello,
>
> in UML kernel I get a long cpu using loop in __getnstimeofday()
> (kernel/time/timekeeping.c:315) in the call of timespec_add_ns(),
> when I left the host kernel suspended to ram for a few hours and resume
> again.
> this is because it seems like the tk->xtime_sec wasn't updated yet, but
> the nsecs were. nsecs can be as high as 8111000111000111000l
>
> the function timespec_add_ns() (include/linux/time.h:266) will call
> __iter_div_u64_rem() which has an optimized loop for the case
> that the dividend is not much bigger as the divisior.
> but this isn't the case for resume from ram on the host kernel.
>
> any ideas how to fix this? is it possible to intercept the resume from
> ram and update the timekeeper->xtime_sec somehow?
> or can the um arch somehow overwrite timespec_add_ns() to always use
> div_u64_rem() instead?

Hmm, does this always happen?
At least on my notebook it did not happen. I've started an UML yesterday
suspended it and after more than 12h it worked fine today.

BTW: Do you see the issue also then freezing UML using the freezer cgroup?
Would be easier to debug. :)

-- 
Thanks,
//richard