Changes between Version 11 and Version 12 of scheduler


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Timestamp:
Sep 23, 2018, 1:10:41 PM (3 years ago)
Author:
alain
Comment:

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  • scheduler

    v11 v12  
    1515ALMOS-MKH defines two types of threads:
    1616 * USER threads are POSIX compliant threads, defined in a given user process. A main thread is always created for an user process. Other threads are created by the pthread_create() syscall.
    17  * KERNEL threads implement kernels services: RPC threads execute the Remote Procedure Calls; DEV threads implement IO channels operations, and IDLE thread is the default (low-power) thread.
     17 * KERNEL threads implement kernels services: RPC threads execute the Remote Procedure Calls; DEV threads implement IO channels operations, and the IDLE thread is the default (low-power) thread.
    1818
    1919From the scheduler point of view, any thread (KERNEL or USER) can be in three states:
     
    4141Finally ALMOS_MKH implements the following scheduling priority :   RPC > DEV > USER > IDLE
    4242
    43 == D) Delayed Context Switches ==
     43== D) Context Switches ==
     44
     45As a general rule, a thread can deschedule when it is holding an high level kernel lock (queuelock or rwlock), but cannot deschedule when it is holding a kernel busy-waiting lock.
    4446
    4547Context switches can have two causes:
    46 The RUNNING thread can explicitly ask to be descheduled, when blocked on a shared resource. Before descheduling, the thread should release all kernel locks.
    47 But the scheduling policy being preemptive,  the RUNNING thread can hold one (or several) kernel lock(s) when receiving a TICK interrupt.
     48The RUNNING thread can explicitly ask to be descheduled, when blocked on a shared resource. Before descheduling, the thread should release all taken kernel busy locks. But the scheduling policy being preemptive,  the RUNNING thread can hold one (or several) kernel lock(s) when receiving a TICK interrupt. Therefore, the
     49busylock_acquire() function disables IRQs before it takes the lock, and the busylock_release() function restore IRQs only after it releases the lock.
    4850
    49 In this situation, ALMOS-MKH implements the following delayed context switch mechanism:
    50  1. The RUNNING thread holding locks continues execution, but the THREAD_FLAG_SCHED is set in the running thread descriptor.
    51  2. All kernel functions used to release kernel locks check this THREAD_FLAG_SCHED. When this flag is set, and the last lock is released by the calling thread, the sched_yield() is immediately executed.
    52 
    53 More generally, ALMOS-MKH supports descheduling of an USER thread that is currently in kernel mode - as a result of a syscall. In other words, syscalls can be interrupted by interrupts signaling the completion of an I/O operations, or by the TICK interrupt, requiring a context switch.
     51On the other hand, ALMOS-MKH supports descheduling of an USER thread that is currently in kernel mode - as a result of a syscall. In other words, some syscalls (not all) can be interrupted by interrupts signaling the completion of an I/O operations, or by the TICK interrupt, requiring a context switch.
    5452
    5553== E) Floating Point Unit ==