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CTXSW(9)                 BSD Kernel Developer's Manual                CTXSW(9)

     mi_switch, cpu_switch -- switch to another light weight process

     mi_switch(struct lwp *cur, struct lwp *new);

     cpu_switch(struct lwp *cur, struct lwp *new);

     The mi_switch() function implements the machine-independent prelude to an
     LWP context switch.  It is called from only a few distinguished places in
     the kernel code as a result of the principle of non-preemptable kernel
     mode execution.  The three major uses of mi_switch() can be enumerated as

           1.   From within tsleep(9) when the current LWP voluntarily relin-
                quishes the CPU to wait for some resource to become available.

           2.   From within preempt(9) when the current LWP voluntarily relin-
                quishes the CPU or when the kernel prepares a return to user-
                mode execution.

           3.   In the signal handling code if a signal is delivered that
                causes an LWP to stop (see issignal(9)).

     mi_switch() records the amount of time the current LWP has been running
     in the LWP structure and checks this value against the CPU time limits
     allocated to the LWP (see getrlimit(2)).  Exceeding the soft limit
     results in a SIGXCPU signal to be posted to the LWP, while exceeding the
     hard limit will cause a SIGKILL.  If newp is NULL, mi_switch() will
     invoke chooseproc(9) to select a new LWP from the system run queue.  If
     the new LWP is the same as the current LWP, then control is returned
     immediately to the LWP, avoiding any unnecessary overhead associated with
     switching in a new LWP.  Otherwise, mi_switch() hands over control to the
     machine-dependent function cpu_switch(), which will perform the actual
     LWP context switch.

     The cpu_switch() function switches the new LWP onto the CPU.  This proce-
     dure is performed by saving the LWP context of the current LWP in its
     process control block (PCB) and restoring the LWP context of the new LWP.
     The address space of the new LWP is activated using pmap_activate() (see
     pmap(9)).  Finally, the new LWP is checked to see if it was previously
     preempted while inside a restartable atomic sequence (see ras(9)).

     These functions must be called with the scheduler lock held (see
     SCHED_LOCK(9)) and at the splsched(9) interrupt protection level.  Both
     functions return with the scheduler lock released.

     These functions return 1 if a context switch was performed to a different
     LWP, 0 otherwise.

     SCHED_LOCK(9), chooseproc(9), pmap(9), ras(9), splsched(9), tsleep(9),

BSD                              May 13, 2003                              BSD