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Project 3

Result

Question 1 - context switches

./test1
This process encounters 10 times context switches.

dmesg

[  372.491580] getrusage successful.
[  372.491581] current->nvcsw: 0
[  372.491581] current->nivcsw: 10
[  372.491582] copy_to_user successful with 16.

Question 2 - entering a wait queue

./test2
123456 (<-input)
This process encounters 4 times context switches.
This process enters a wait queue 1 times.
I love my home.
I love my home.
I love my home.
I love my home.
I love my home.
I love my home.
This process enters a wait queue 1 times.

dmesg

[  407.226048] getrusage successful.
[  407.226050] current->nvcsw: 1
[  407.226050] current->nivcsw: 3
[  407.226051] copy_to_user successful with 16.
[  407.226123] get wait_queue successful.
[  407.226124] copy_to_user successful with 8.
[  407.226154] get wait_queue successful.
[  407.226155] copy_to_user successful with 8.

Problem Set

state of the process

new, ready, run, waiting, terminated wake_up(), preempt(), schedule(), yield(), terminate()

wake_up, schedule

linux-kernel-slides.pdf#page.311 | bootlin.com

trace code

nvcsw => voluntary_ctxt_switches nivcsw => nonvoluntary_ctxt_switches

new process

No matter syscall: fork, vfork, clone or function: do_fork, _do_fork, kernel_thread, copy_process. Will check flags and copy mem.

copy_mm

copy_mm initialize task_struct values: nvcsw, nivcsw.

ps. CONFIG_DETECT_HUNG_TASK enable: checking TASK_UNINTERRUPTIBLE (aka. "D" state) which will lead to unable receiving signal.

copy_thread_tls

function under ./arch/x86/kernel/process_64.c or ./arch/x86/kernel/process_32.c: copy_thread_tls initialize self define variable inside task_struct my_wait_queue

CONFIG_HAVE_COPY_THREAD_TLS ./arch/Kconfig#L804: Architecture provides copy_thread_tls to accept tls argument via normal C parameter passing, rather than extracting the syscall argument from pt_regs.

TLS: Thread Local Storage Thread Local Storage (TLS) | Microsoft Docs

process been context switches

kernel/sched/core.c

schedule => pick_next_task (sched_class (rt_sched_class, fair_sched_class, idle_sched_class))

++*switch_count

further reading: preempt, schedule, pick_next_task, getrusage, accumulate_thread_rusage

Process got out of (into) wait queue

TO default_wake_function calling chain

include/linux/wait.h kernel/exit.c

syscall: wait4, waitid, waitpid => kernel_waitid, kernel_wait4 => do_wait => ... => init_waitqueue_func_entry, init_waitqueue_entry (wake_up handler)

FROM default_wake_function calling chain

kernel/sched/core.c

default_wake_function => try_to_wake_up => ttwu_queue => ttwu_do_activate => ttwu_do_wakeup

ttwu_do_wakeup

  1. check state: TASK_RUNNING
  2. manipulate my_wait_queue
example: function handler inside IO wait

drivers/infiniband/hw/hfi1/iowait.h