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.Dd July 21, 2007 .Dt MI_SWITCH 9 .Os .Sh NAME .Nm mi_switch .Nd machine independent context switch prelude .Sh SYNOPSIS .Ft int .Fn mi_switch "struct lwp *l" .Sh DESCRIPTION The .Fn 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 .Fn mi_switch can be enumerated as follows: l -enum -offset indent t From within .Xr cv_wait 9 and associated methods when the current LWP voluntarily relinquishes the CPU to wait for some resource to become available. t From within .Xr preempt 9 when the current LWP voluntarily relinquishes the CPU or when the kernel prepares a return to user-mode execution. t In the signal handling code if a signal is delivered that causes an LWP to stop
q see Xr issignal 9 . .El
p .Fn 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
q see Xr getrlimit 2 . Exceeding the soft limit results in a .Dv SIGXCPU signal to be posted to the LWP, while exceeding the hard limit will cause a .Dv SIGKILL .
p Unless .Fa l->l_switchto is not .Dv NULL , .Fn mi_switch will call .Fn sched_nextlwp to select a new LWP from the scheduler's runqueue structures. If no runnable LWP is found, the idle LWP is used. If the new LWP is not equal to the current one, .Fn mi_switch will hand over control to the machine-dependent function .Xr cpu_switchto 9 to switch to the new LWP.
p .Fn mi_switch has to be called with the LWP lock held (through calling .Fn lwp_lock first) and at the .Xr splsched 9 interrupt protection level. It returns with the LWP lock released. .Sh RETURN VALUES .Fn mi_switch returns 1 if a context switch was performed to a different LWP, 0 otherwise. .Sh SEE ALSO .Xr condvar 9 , .Xr cpu_switchto 9 , .Xr csf 9 , .Xr pmap 9 , .Xr ras 9 , .Xr sched_4bsd 9 , .Xr splsched 9