[uml-devel] Building/runing UML in SMP mode

[uml-devel] Building/runing UML in SMP mode

[Paul Menage]

For a while now (since tt mode was dropped and skas mode became the
only option?)  it looks as though UML has been UP-only on x86-64. And
even though the configs appear to allow building SMP on x86-32, there
are things like the panic() call in kernel/smp.c:idle_thread() which
imply that it won't work too well in 32-bit mode either.

For testing cgroups/cpusets changes we'd find it useful to be able to
run UML in SMP mode. Is anyone working on this, or can point us at a
list of the major things that need to be done to make SMP UML
practical?

Thanks,

Paul

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Re: [uml-devel] Building/runing UML in SMP mode

[Jeff Dike]

On Thu, Jun 11, 2009 at 12:02:56PM -0700, Paul Menage wrote:
> For a while now (since tt mode was dropped and skas mode became the
> only option?)  it looks as though UML has been UP-only on x86-64. And
> even though the configs appear to allow building SMP on x86-32, there
> are things like the panic() call in kernel/smp.c:idle_thread() which
> imply that it won't work too well in 32-bit mode either.
> 
> For testing cgroups/cpusets changes we'd find it useful to be able to
> run UML in SMP mode. Is anyone working on this, or can point us at a
> list of the major things that need to be done to make SMP UML
> practical?

SMP has been problematic because of ptrace.  Ironically, despite the
horridness of tt mode, that was where SMP was easiest to do.

The problem with SMP on skas0 is the need to detach a userspace
process from one virtual CPU process and attach it to another whenever
the associated UML process is migrated from one CPU to another.

I actually had this somewhat working, but the code was a horror show,
with much nastiness about ignoring the signals that are needed in
order to prevent a detached process from running.

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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Re: [uml-devel] Building/runing UML in SMP mode

[Paul Menage]

On Mon, Jun 15, 2009 at 6:03 PM, Jeff Dike<jdike@addtoit.com> wrote:
>
> The problem with SMP on skas0

Does that mean that it works more easily on skas3?

> I actually had this somewhat working, but the code was a horror show,
> with much nastiness about ignoring the signals that are needed in
> order to prevent a detached process from running.
>

So it's probably not on the horizon any time soon then?

Thanks,

Paul

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Re: [uml-devel] Building/runing UML in SMP mode

[Jeff Dike]

On Mon, Jun 15, 2009 at 06:22:46PM -0700, Paul Menage wrote:
> On Mon, Jun 15, 2009 at 6:03 PM, Jeff Dike<jdike@addtoit.com> wrote:
> >
> > The problem with SMP on skas0
> 
> Does that mean that it works more easily on skas3?

Yup.  That completely avoids ptrace issues.  I think things should
just about work at that point.  When you have multiple threads on
different CPUs running simultaneously, they will just switch
themselves into the appropriate address space.  I can't think of any
problems with this offhand.

> > I actually had this somewhat working, but the code was a horror show,
> > with much nastiness about ignoring the signals that are needed in
> > order to prevent a detached process from running.
> >
> 
> So it's probably not on the horizon any time soon then?

No - I haven't done much with UML in a while...

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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