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39 .Nd switch to another thread context
52 function implements the machine independent prelude to a thread context
54 It is called from only a few distinguished places in the kernel
55 code as a result of the principle of non-preemptable kernel mode execution.
56 The various major uses of
58 can be enumerated as follows:
59 .Bl -enum -offset indent
61 From within a function such as
66 when the current thread
67 voluntarily relinquishes the CPU to wait for some resource or lock to become
71 (e.g.\& a system call, device interrupt)
72 when the kernel prepares a return to user-mode execution.
74 typically handled by machine dependent trap-handling code after detection
75 of a change in the signal disposition of the current process, or when a
76 higher priority thread might be available to run.
78 communicated by the machine independent scheduling routines by calling
82 In the signal handling code
85 if a signal is delivered that causes a process to stop.
89 and control of the processor can be passed to the next runnable thread.
92 .Xr thread_suspend_check 9
93 where a thread needs to stop execution due to the suspension state of
94 the process as a whole.
98 records the amount of time the current thread has been running in the
99 process structures and checks this value against the CPU time limits
100 allocated to the process
103 Exceeding the soft limit results in a
105 signal to be posted to the process, while exceeding the hard limit will
109 If the thread is still in the
113 will put it back onto the run queue, assuming that
114 it will want to run again soon.
115 If it is in one of the other
116 states and KSE threading is enabled, the associated
118 will be made available to any higher priority threads from the same
119 group, to allow them to be scheduled next.
121 After these administrative tasks are done,
123 hands over control to the machine dependent routine
125 which will perform the actual thread context switch.
128 first saves the context of the current thread.
131 to determine which thread to run next.
132 Finally, it reads in the saved context of the new thread and starts to
133 execute the new thread.
138 except that it does not save the context of the old thread.
139 This function is useful when the kernel does not have an old thread
140 context to save, such as when CPUs other than the boot CPU perform their
141 first task switch, or when the kernel does not care about the state of the
142 old thread, such as in
144 when the kernel terminates the current thread and switches into a new
149 all of these functions must be called with the