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.Dd January 9, 2023
.Dt MI_SWITCH 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switch ,
.Nm cpu_throw
.Nd switch to another thread context
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
.Ft void
.Fn mi_switch "int flags"
.Ft void
.Fn cpu_switch "struct thread *oldtd" "struct thread *newtd" "struct mtx *lock"
.Ft void
.Fn cpu_throw "struct thread *oldtd" "struct thread *newtd"
.Sh DESCRIPTION
The
.Fn mi_switch
function implements the machine-independent prelude to a thread context
switch.
It is the single entry point for every context switch and is called from only
a few distinguished places in the kernel.
The context switch is, by necessity, always performed by the switched thread,
even when the switch is initiated from elsewhere; e.g. preemption requested via
Inter-Processor Interrupt (IPI).
.Pp
The various major uses of
.Fn mi_switch
can be enumerated as follows:
.Bl -enum -offset indent
.It
From within a function such as
.Xr sleepq_wait 9
or
.Fn turnstile_wait
when the current thread
voluntarily relinquishes the CPU to wait for some resource or lock to become
available.
.It
Involuntary preemption due to arrival of a higher-priority thread.
.It
At the tail end of
.Xr critical_exit 9 ,
if preemption was deferred due to the critical section.
.It
Within the TDA_SCHED AST handler, when rescheduling before the return to
usermode was requested.
There are several reasons for this, a notable one coming from
.Fn sched_clock
when the running thread has exceeded its time slice.
.It
In the signal handling code
(see
.Xr issignal 9 )
if a signal is delivered that causes a process to stop.
.It
In
.Fn thread_suspend_check
where a thread needs to stop execution due to the suspension state of
the process as a whole.
.It
In
.Xr kern_yield 9
when a thread wants to voluntarily relinquish the processor.
.El
.Pp
The
.Va flags
argument to
.Fn mi_switch
indicates the context switch type.
One of the following must be passed:
.Bl -tag -offset indent -width "SWT_REMOTEWAKEIDLE"
.It Dv SWT_OWEPREEMPT
Switch due to delayed preemption after exiting a critical section.
.It Dv SWT_TURNSTILE
Switch after propagating scheduling priority to the owner of a resource.
.It Dv SWT_SLEEPQ
Begin waiting on a
.Xr sleepqueue 9 .
.It Dv SWT_RELINQUISH
Yield call.
.It Dv SWT_NEEDRESCHED
Rescheduling was requested.
.It Dv SWT_IDLE
Switch from the idle thread.
.It Dv SWT_IWAIT
A kernel thread which handles interrupts has finished work and must wait for
interrupts to schedule additional work.
.It Dv SWT_SUSPEND
Thread suspended.
.It Dv SWT_REMOTEPREEMPT
Preemption by a higher-priority thread, initiated by a remote processor.
.It Dv SWT_REMOTEWAKEIDLE
Idle thread preempted, initiated by a remote processor.
.It Dv SWT_BIND
The running thread has been bound to another processor and must be switched
out.
.El
.Pp
In addition to the switch type, callers must specify the nature of the
switch by performing a bitwise OR with one of the
.Dv SW_VOL
or
.Dv SW_INVOL
flags, but not both.
Respectively, these flags denote whether the context switch is voluntary or
involuntary on the part of the current thread.
For an involuntary context switch in which the running thread is
being preempted, the caller should also pass the
.Dv SW_PREEMPT
flag.
.Pp
Upon entry to
.Fn mi_switch ,
the current thread must be holding its assigned thread lock.
It may be unlocked as part of the context switch.
After they have been rescheduled and execution resumes, threads will exit
.Fn mi_switch
with their thread lock unlocked.
.Pp
.Fn mi_switch
records the amount of time the current thread has been running before handing
control over to the scheduler, via
.Fn sched_switch .
After selecting a new thread to run, the scheduler will call
.Fn cpu_switch
to perform the low-level context switch.
.Pp
.Fn cpu_switch
is the machine-dependent function that performs the actual switch from the
running thread
.Fa oldtd
to the chosen thread
.Fa newtd .
First, it saves the context of
.Fa oldtd
to its Process Control Block,
.Po
PCB
.Vt struct pcb
.Pc ,
pointed at by
.Va oldtd->td_pcb .
The function then updates important per-CPU state such as the
.Dv curthread
variable, and activates
.Fa newtd\&'s
virtual address space using its associated
.Xr pmap 9
structure.
Finally, it reads in the saved context from
.Fa newtd\&'s
PCB.
CPU instruction flow continues in the new thread context, on
.Fa newtd\&'s
kernel stack.
The return from
.Fn cpu_switch
can be understood as a completion of the function call initiated by
.Fa newtd
when it was previously switched out, at some point in the distant (relative to
CPU time) past.
.Pp
The
.Fa mtx
argument to
.Fn cpu_switch
is used to pass the mutex which will be stored as
.Fa oldtd\&'s
thread lock at the moment that
.Fa oldtd
is completely switched out.
This is an implementation detail of
.Fn sched_switch .
.Pp
.Fn cpu_throw
is similar to
.Fn 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
.Xr thread_exit 9
when the kernel terminates the current thread and switches into a new
thread,
.Fa newtd .
The
.Fa oldtd
argument is unused.
.Sh SEE ALSO
.Xr critical_exit 9 ,
.Xr issignal 9 ,
.Xr kern_yield 9 ,
.Xr mutex 9 ,
.Xr pmap 9 ,
.Xr sleepqueue 9 ,
.Xr thread_exit 9