Re: [uml-devel] [PATCH v6] um: Add a high resolution timer subsystem

[Thomas Meyer <thomas@m3y3r.de>]

Am 11.05.2015 9:42 nachm. schrieb Anton Ivanov <anton.ivanov@kot-begemot.co.uk>:
>
> On 11/05/15 18:41, Thomas Meyer wrote: 
> > Am 11.05.2015 7:31 nachm. schrieb Anton Ivanov <anton.ivanov@kot-begemot.co.uk>: 
> >> On 11/05/15 18:20, Anton Ivanov wrote: 
> >>> On 11/05/15 18:00, Thomas Meyer wrote: 
> >>>> Hi, 
> >>>> 
> >>>> maybe there is a bug in how the timers are created for all user space processes. 
> >>>> In the latest patch I use os__timer_remain for the initial interval. 
> >>>> The idea was to launch all timers at the same time. But I now think this can never work using relative times, especially when os__timer_remain returns 0, then the new timer is never launched. 
> >>> That is not a bad idea, just requires a more complex check in setting up 
> >>> interval so it is reset to default when a zero value or value > HZ is 
> >>> passed. 
> >>> 
> >>> 
> >>>> That may explain the hangs you see. 
> >>>>      You could try to replace os__timer_remain with the current HZ value in nanoseconds. 
> >>> I am going to adjust time.c instead as above. 
> >> Both approaches fail - the check (that is expected actually, I did not 
> >> think properly here - it does not cover the stub in kernel/skas) and the 
> >> setting of data to the correct initial values. 
> >> 
> >> Also, if it was just not starting timers it would have been possible to 
> >> beat the process with USR2 on the head until its morale improves. Well, 
> >> "while true; do kill -USR2 PID ; done" does not change the behavior, it 
> >> still hangs and is still possible to terminate the errant process by 
> >> sending SIGSEGV to the UML thread which is shown as 100% CPU and in R state. 
> > Yes, sending USR2 should do the same. Correct. 
> > 
> >> Problem is somewhere else. I am surprised it worked correctly in the 
> >> original one as there for 100% flat CPU usage it should have converged 
> >> to a behavior which is similar to what we have now. 
> > Mhh. Strange. Any hints how I can reproduce this? 
> > 
> > I need to see the same behaviour, i.e. the hang on my machine to begin to understand what is going on here. 
>
> Start off with a Debian image of let's say wheezy and hit it with a 
> dist-upgrade to the next release. This produces enough IO+CPU at the 
> same time to trigger it. I get it 100% reproducible every time it tries 
> to unpack the first couple of packages. 

Okay, I'll try that!

>
> I tried to trigger backtraces but could not get anything informative. 
> The old trick of hitting the kernel with SIGILL which worked very nicely 
> in older kernel versions no longer works :( 

You may can trigger an backtrace via uml_console.

>
> > 
> > Thanks for testing Anton. 
>
> I wish I had more time at the moment to debug it and fix it :( We are 
> nearly there and it is worth it. 

Yes, same here.
But we will finish this!

>
> A. 
>
> > 
> > 
> >> A. 
> >> 
> >>> A. 
> >>> 
> >>>> Kind regards 
> >>>> Thomad 
> >>>> 
> >>>> A 
> >>> [snip] 
> >>> 
> >>> A. 
> >>> 
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