MI_SWITCH(9)

💅 MI_SWITCH — bestie edition 💅

okay so we're in the kernel developer's manual, and today's main character is MI_SWITCH(9) — aka mi_switch, cpu_switch, and cpu_throw, the trio whose whole entire job is switching to another thread context. no cap. 🦋

✨ NAME ✨

mi_switch, cpu_switch, cpu_throw — switch to another thread context

🌸 SYNOPSIS 🌸

#include <sys/param.h>
#include <sys/proc.h>

void
mi_switch(void);

void
cpu_switch(void);

void
cpu_throw(void);

💖 DESCRIPTION 💖

so the mi_switch() function is literally the machine independent prelude to a thread context switch. it does NOT get called from just anywhere, bestie — only from a few distinguished spots in the kernel code, and that's because of the principle of non-preemptable kernel mode execution. (iykyk 👑)

the various major uses of mi_switch can be enumerated as follows:

1. from within a function such as cv_wait(9), mtx_lock(9), or tsleep(9) when the current thread voluntarily relinquishes the CPU to wait for some resource or lock to become available. (it's giving "i'll be back, gonna go touch grass" 🌱)
2. after handling a trap (e.g. a system call, device interrupt) when the kernel prepares a return to user-mode execution. this case is typically handled by machine dependent trap-handling code after detection of a change in the signal disposition of the current process, or when a higher priority thread might be available to run. that latter event is communicated by the machine independent scheduling routines by calling the machine defined need_resched().
3. in the signal handling code (see issignal(9)) if a signal is delivered that causes a process to stop. 🛑
4. when a thread dies in thread_exit(9) and control of the processor can be passed to the next runnable thread. rip to that thread, but i'm different 💀
5. in thread_suspend_check(9) where a thread needs to stop execution due to the suspension state of the process as a whole.

okay so now mi_switch() gets ✨bookkeeping✨: it records the amount of time the current thread has been running in the process structures, and it checks that value against the CPU time limits allocated to the process (see getrlimit(2)).

exceeding the soft limit results in a SIGXCPU signal being posted to the process, while exceeding the hard limit will cause a SIGKILL. 😭 dramatic but true, bestie.

if the thread is still in the TDS_RUNNING state, mi_switch() will put it back onto the run queue, assuming that it will want to run again soon. and if it's in one of the other states and KSE threading is enabled, the associated KSE will be made available to any higher priority threads from the same group, to allow them to be scheduled next. generous queen behavior tbh 💅

after these administrative tasks are done, mi_switch() hands over control to the machine dependent routine cpu_switch(), which will perform the actual thread context switch. the way that the handoff happens?? obsessed.

🎀 and now, the actual switchers 🎀

cpu_switch() first saves the context of the current thread. next, it calls choosethread() to determine which thread to run next. finally, it reads in the saved context of the new thread and starts to execute the new thread. ✨ main character energy for whichever thread gets picked, fr fr.

cpu_throw() is similar to cpu_switch() except that it does not save the context of the old thread. this function is useful when the kernel does not have an old thread context to save — such as when CPUs other than the boot CPU perform their first task switch — or when the kernel does not care about the state of the old thread, such as in thread_exit() when the kernel terminates the current thread and switches into a new thread. no looking back, periodt. 🦋

⚠️ warning bestie, do not skip this one: to protect the runqueue(9), all of these functions must be called with the sched_lock mutex held. hold the lock. hold it. 🔒

🌈 SEE ALSO 🌈

• cv_wait(9)
• issignal(9)
• mutex(9)
• runqueue(9)
• tsleep(9)
• wakeup(9)

k that's the whole page, bestie 💜 this one came from the Debian crew, dated November 24, 1996, and it's filed under the name MI_SWITCH(9) — go forth and switch those contexts responsibly 🫶✨