The behavior of scheduler in L4Linux on ARM architecture

[Adam Lackorzynski]

Hi,

On Tue May 19, 2009 at 20:06:32 +0800, Sean wrote:
> On Tue, May 12, 2009 at 4:38 AM, Adam Lackorzynski <
> adam at os.inf.tu-dresden.de> wrote:
> > >    1. In L4Linux, the process/thread will map to several L4 Threads. How
> > the
> > >    scheduler in the Linux Server considers user processes in L4Linux
> > whether to
> > >    execute in L4 system ? According to the document, does L4Linux
> > scheduler
> > >    only control the user process whether block or not?
> >
> > The document is a bit out-dated regarding this area. The scheduler in
> > L4Linux does nothing different than the one in Linux.
> >
> > >    2. Based on the L4Linux source code from currently SVN, the behavior
> > of
> > >    schedule() function (kernel/sched.c line 4546) is
> > >    => If next != prev, call context_switch() function
> > >    => In context_switch() function (kernel/sched.c Line 2649), it calls
> > >    architecture dependent function switch_to()
> > >    => In switch_to() function (arch/l4/include/asm/arch-arm/system.h Line
> > >    222), it calls l4x_switch_to() function and   __switch_to() function
> > >    => In l4x_switch_to() function (arch/l4/kernel/arch-arm/dispatch.c
> > Line
> > >    170), it logs the message to Trace Buffer of Fiasco and set
> > >    l4x_current_process
> > >    => In __switch_to() function (arch/l4/kernel/arch-arm/switch_to.S Line
> > >    5), it store the registers to CPU context of thread_info and load all
> > regs
> > >    saved previously
> > >
> > >    Based on the source code analysis, it seems that when the L4Linux
> > >    scheduler decide to do the context switch, the architecture dependent
> > >    function __switch_to() will do the register restore and load.
> > According to
> > >    the document, I think the behavior here should set the corresponding
> > thread
> > >    status of prev and next user process to block or unblock these L4
> > threads.
> > >    But here it does low level register restore and reload operations to
> > change
> > >    the current running process directly. It confuses me. Can anyone give
> > me any
> > >    ideas to address this?
> >
> > switch_to switches the kernel stack and does nothing with the user state
> > of the process. Nowadays only one L4 thread is active at a time, i.e.
> > the one whose context is currently active in L4Linux. All others are
> > blocked until the Linux scheduler decides to switch to those contexts
> > and they're ready to be released.
> 
> 
> I had found some articles in mailing list to describe the L4 scheduler.
> http://os.inf.tu-dresden.de/pipermail/l4-hackers/2007/003342.html
> And I already traced the source code appeared in these articles.
> 
> But I still had some questions here.
> 
> 1. When L4Linux scheduler decides to switch to Task A, the
> l4x_user_dispatcher() will execute by restoring registers in function
> switch_to(). Is this statement true?

switch_to() in the Linux kernel does switch the kernel context, i.e. it
switches the kernel stack. It does not touch any user state (registers).

>     In l4x_user_dispatcher(), it runs l4x_spawn_cpu_thread(0, p, t) to
> create a new task first and get into a infinite loop. I am confused at the
> thread status of Fiasco here.
>     Is the task created by l4ts_allocate_task2() a L4 thread ?

Yes, and also an address space (task).

>     Is
> l4x_user_dispatcher() running in L4Linux main thread, which is l4lx.cpu0 ?

Yes.

>     Can you give me any idea about the relationship between l4lx.cpu0 thread
> and the thread corresponds to L4Linux user process?

l4lx.cpu0 is executing the Linux kernel code and the thread of the user
processes are executing the user code. l4lx.cpu0 is the fault handler of
the user threads and is executing kernel code on behalf of the user
code.

> 2. What's the difference between task and thread in L4Linux?

A task is basically an address space in which multiple threads can run
in. It's the same meaning as used in L4.




Adam
-- 
Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/