Saturday, April 27, 2013

Lab4 Preemptive Multitasking

[Preface] A Hope That My Crude Remarks May Draw Forth By Abler People

GitHub: MIT-6.828-Adventure-Lab4

  • Part A: Multiprocessor Support and Cooperative Multitasking.
    • Initialize hardware support.
      • Main goal: Make every CPUs have its own independent data.
      • Booting CPU triggers the bootaps() to set up each CPU by env_init_percpu() through mpentry.S to call mp_main().
      • ma_main() triggers
        • env_init_percpu
          • trap_init_percpu() setup each CPU's own TSS and stack allocated from mem_init_mp()
            • ltr(tss)
            • lidt
        • trap_init_percpu()
          • setup es/ds/fs/gs/fs/ss
          • setup CS by ljmp
    • Big Kernel Lock setup to ensure only one trap to capable to enter the kernel mode in the same time between all CPUs.
    • Implement Round-Robin scheduling.
  • Part B: Copy-on-Write Fork. 
    • I must say this part is very very interesting!!!
      • user mode page fault handler
      • The translation between UXSTACK, Kernel stack, and normal user stack.
    • The guide on Lab4 is very very detailed, I hope U could enjoy this part!
    • Note: Difference between faultalloc.c and faultallocbad.c
      • faultallocbad.c trigger user_mem_assert under kernel mode, so it can't print out.
      • faultalloc.c trigger 3 times page fault under user mode
        • 1 by deadbeef printing.
        • 2 by cafebffe printing.(contains recursive page fault)
  • Part C: Preemptive Multitasking and Inter-Process communication (IPC).
    • Avoid one user process keep occupying CPU if it likes.
    • Enable timer interrupt as the Force in Jedi to make Process release CPU.
    • IPC here is applied in Blocking Send and Blocking Receive.
      • It could be used as the basic lock mechanism, too.
    • This part could be the Integration system test for lab 1~4, for interactions between kernel and users. :)
  • Some questions assignment after exercises.