Re: [uml-devel] UML hangs with hrtimer test module

[Joel F. <agn...@gm...>]

On Wed, Mar 28, 2018 at 11:04 PM, Geert Uytterhoeven
<ge...@li...> wrote:
> On Thu, Mar 29, 2018 at 12:35 AM, Richard Weinberger <ri...@no...> wrote:
>> Am Donnerstag, 29. März 2018, 00:19:39 CEST schrieb Joel Fernandes:
>>> On Wed, Mar 28, 2018 at 6:19 AM, Richard Weinberger <ri...@no...> wrote:
>>> > Am Mittwoch, 28. März 2018, 15:11:29 CEST schrieb Geert Uytterhoeven:
>>> >> On Wed, Mar 28, 2018 at 12:28 PM, Joel Fernandes <agn...@gm...>
>>> > wrote:
>>> >> >         while(release_now == 0);
>>> >>
>>> >> while (release_now == 0)
>>> >>         cpu_relax();
>>> >
>>> > Not sure whether a cpu_relax() fixes the problem.
>>> > I guess the root of the problem is that UML is UP and non-preemptive.
>>> > Therefore the loop is never interrupted.
>>> > To verify I asked for the full source.
>>> >
>>>
>>> cpu_relax actually worked!
>>
>> Interesting.
>>
>>> Any thoughts on why it helps? Even if its non-preemptive, I did
>>> receive the timer interrupt, so I expected the variable to be set.
>>
>> Timers trigger also with preempt off, I forgot...
>> I think the cpu_relax() issues internally a barrier such that the
>> release_now variable is read again.
>> Can you try barrier() instead of cpu_relax()? I bet it works too.
>> Same if you mark release_now as volatile.
>
> Without cpu_relax()/barrier()/volatile, the compiler can assume release_now
> never changes, and thus may "optimize" the loop to an infinite loop.
>

Thanks a lot! I am wondering why the same compiler works when running
the test for a regular image. Maybe different compiler flags. Anyway
good to learn this.

Also one more slightly OT question, why is UML only doing UP ? Is it
extremely hard to do SMP for UML?

I also happen to notice Qemu has one thread per emulated core...

thanks,

- Joel