[uml-devel] preemption

[uml-devel] preemption

[Sam King]

Hello!

I am trying to understand how the UML kernel preempts
guest user mode applications in SKAS mode.  I am using
version 2.4.20 of the guest kernel.

From what I can tell, when the guest kernel takes a
timer interrupt (SIGALRM) it eventually updates the
tsk->need_resched parameter for processes that have
been running for too long.  However, I do not see
where the actual switch takes place except in the
interrupt_end() function which is only called as a
result of guest application system calls and signals. 
Since I am sure that the guest kernel supports
preemption, I must be missing something.

Thank you for you help.

--Sam


	
		
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Re: [uml-devel] preemption

[Jeff Dike]

On Tue, May 18, 2004 at 06:47:26AM -0700, Sam King wrote:
> >From what I can tell, when the guest kernel takes a
> timer interrupt (SIGALRM) it eventually updates the
> tsk->need_resched parameter for processes that have
> been running for too long.  However, I do not see
> where the actual switch takes place except in the
> interrupt_end() function which is only called as a
> result of guest application system calls and signals. 

Exactly.  Every kernel entrance is either a system call or signal (a UML
signal, not a process signal, BTW), so there is a preemption opportunity
at every return to userspace.

				Jeff


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Re: [uml-devel] preemption

[BlaisorBlade]

Alle 20:38, martedì 18 maggio 2004, Jeff Dike ha scritto:
> On Tue, May 18, 2004 at 06:47:26AM -0700, Sam King wrote:
> > >From what I can tell, when the guest kernel takes a
> >
> > timer interrupt (SIGALRM) it eventually updates the
> > tsk->need_resched parameter for processes that have
> > been running for too long.  However, I do not see
> > where the actual switch takes place except in the
> > interrupt_end() function which is only called as a
> > result of guest application system calls and signals.
>
> Exactly.  Every kernel entrance is either a system call or signal (a UML
> signal, not a process signal, BTW), so there is a preemption opportunity
> at every return to userspace.
But when you say "UML signal" (I guess a signal between the host UML threads) 
you mean even a timer interrupt, right? Otherwise I don't see how a 
non-syscalling process (a true CPU-hog, like SETI@home or such things) could 
be interrupted.
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729



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