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39 .Dd November 24, 1996
40 .Dt MI_SWITCH 9
41 .Os
42 .Sh NAME
43 .Nm mi_switch ,
44 .Nm cpu_switch ,
45 .Nm cpu_throw
46 .Nd switch to another thread context
47 .Sh SYNOPSIS
48 .In sys/param.h
49 .In sys/proc.h
50 .Ft void
51 .Fn mi_switch "void"
52 .Ft void
53 .Fn cpu_switch "void"
54 .Ft void
55 .Fn cpu_throw "void"
56 .Sh DESCRIPTION
57 The
58 .Fn mi_switch
59 function implements the machine independent prelude to a thread context
60 switch.
61 It is called from only a few distinguished places in the kernel
62 code as a result of the principle of non-preemptable kernel mode execution.
63 The various major uses of
64 .Nm
65 can be enumerated as follows:
66 .Bl -enum -offset indent
67 .It
68 From within a function such as
69 .Xr cv_wait 9 ,
70 .Xr mtx_lock ,
71 or
72 .Xr tsleep 9
73 when the current thread
74 voluntarily relinquishes the CPU to wait for some resource or lock to become
75 available.
76 .It
77 After handling a trap
78 (e.g.\& a system call, device interrupt)
79 when the kernel prepares a return to user-mode execution.
80 This case is
81 typically handled by machine dependent trap-handling code after detection
82 of a change in the signal disposition of the current process, or when a
83 higher priority thread might be available to run.
84 The latter event is
85 communicated by the machine independent scheduling routines by calling
86 the machine defined
87 .Fn need_resched .
88 .It
89 In the signal handling code
90 (see
91 .Xr issignal 9 )
92 if a signal is delivered that causes a process to stop.
93 .It
94 When a thread dies in
95 .Xr thread_exit 9
96 and control of the processor can be passed to the next runnable thread.
97 .It
98 In
99 .Xr thread_suspend_check 9
100 where a thread needs to stop execution due to the suspension state of
101 the process as a whole.
102 .El
103 .Pp
104 .Fn mi_switch
105 records the amount of time the current thread has been running in the
106 process structures and checks this value against the CPU time limits
107 allocated to the process
108 (see
109 .Xr getrlimit 2 ) .
110 Exceeding the soft limit results in a
111 .Dv SIGXCPU
112 signal to be posted to the process, while exceeding the hard limit will
113 cause a
114 .Dv SIGKILL .
115 .Pp
116 If the thread is still in the
117 .Dv TDS_RUNNING
118 state,
119 .Fn mi_switch
120 will put it back onto the run queue, assuming that
121 it will want to run again soon.
122 If it is in one of the other
123 states and KSE threading is enabled, the associated
124 .Em KSE
125 will be made available to any higher priority threads from the same
126 group, to allow them to be scheduled next.
127 .Pp
128 After these administrative tasks are done,
129 .Fn mi_switch
130 hands over control to the machine dependent routine
131 .Fn cpu_switch ,
132 which will perform the actual thread context switch.
133 .Pp
134 .Fn cpu_switch
135 first saves the context of the current thread.
136 Next, it calls
137 .Fn choosethread
138 to determine which thread to run next.
139 Finally, it reads in the saved context of the new thread and starts to
140 execute the new thread.
141 .Pp
142 .Fn cpu_throw
143 is similar to
144 .Fn cpu_switch
145 except that it does not save the context of the old thread.
146 This function is useful when the kernel does not have an old thread
147 context to save, such as when CPUs other than the boot CPU perform their
148 first task switch, or when the kernel does not care about the state of the
149 old thread, such as in
150 .Fn thread_exit
151 when the kernel terminates the current thread and switches into a new
152 thread.
153 .Pp
154 To protect the
155 .Xr runqueue 9 ,
156 all of these functions must be called with the
157 .Va sched_lock
158 mutex held.
159 .Sh SEE ALSO
160 .Xr cv_wait 9 ,
161 .Xr issignal 9 ,
162 .Xr mutex 9 ,
163 .Xr runqueue 9 ,
164 .Xr tsleep 9 ,
165 .Xr wakeup 9