The behavior of scheduler in L4Linux on ARM architecture

Adam Lackorzynski adam at
Mon May 11 22:38:27 CEST 2009


On Sat May 09, 2009 at 15:29:02 +0800, Sean 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.

Adam                 adam at

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