NAME

cpu_need_resched — context switch notification

SYNOPSIS

#include <sys/cpu.h>

void
cpu_need_resched(struct cpu_info *ci, struct lwp *l, int flags);

DESCRIPTION

The cpu_need_resched() function is the machine-independent interface for the scheduler to notify machine-dependent code that a context switch from the current LWP l, on the cpu ci, is required. This event may occur if a higher priority LWP appears on the run queue or if the current LWP has exceeded its time slice. l is the last LWP observed running on the CPU. It may no longer be running, as cpu_need_resched() can be called without holding scheduler locks.

If the RESCHED_KPREEMPT flag is specified in flags and __HAVE_PREEMPTION C pre-processor macro is defined in <machine/intr.h>, machine-dependent code should make a context switch happen as soon as possible even if the CPU is running in kernel mode. If the RESCHED_KPREEMPT flag is not specified, then RESCHED_UPREEMPT is specified instead.

If the RESCHED_IDLE flag is specified in flags, the last thread observed running on the CPU was the idle LWP.

If RESCHED_REMOTE flag is specified in flags, the request is not for the current CPU. The opposite also holds true. If ci is not the current processor, cpu_need_resched() typically issues an inter processor call to the processor to make it notice the need of a context switch as soon as possible.

cpu_need_resched() is always called with kernel preemption disabled.

Typically, the cpu_need_resched() function will perform the following operations:

• Set a per-processor flag which is checked by userret(9) when returning to user-mode execution.
• Post an asynchronous software trap (AST).
• Send an inter processor interrupt to wake up cpu_idle(9) and/or force an user process across the user/kernel boundary, thus making a trip through userret().

SEE ALSO

sched_4bsd(9), userret(9)