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

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Date:	Mon, 20 Oct 2014 11:51:33 +0200
From:	Thomas Meyer <thomas@...3r.de>
To:	Richard Weinberger <richard.weinberger@...il.com>
Cc:	user-mode-linux-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [uml-devel] [UM] Long loop in __getnsdayoftime() after resume
 from ram

Am 20.10.2014 10:27 schrieb Richard Weinberger <richard.weinberger@...il.com>:
>
> On Sun, Oct 19, 2014 at 2:39 PM, Thomas Meyer <thomas@...3r.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?

Yes, my single core system seems to trigger this every time after resume from ram.

> 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?

I'm not sure what do you mean by this. Do I need to enable some special configs for this in the host or uml kernel?

> Would be easier to debug. :) 
>
> -- 
> Thanks, 
> //richard