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Manual Pages  — SWI


swi_add, swi_remove, swi_sched – register and schedule software interrupt handlers



#include <sys/param.h>
#include <sys/bus.h>
#include <sys/interrupt.h>
extern struct intr_event *tty_intr_event;
extern struct intr_event *clk_intr_event;
extern void *vm_ih;

swi_add(struct intr_event **eventp, const char *name, driver_intr_t handler, void *arg, int pri, enum intr_type flags, void **cookiep);

swi_remove(void *cookie);

swi_sched(void *cookie, int flags);


These functions are used to register and schedule software interrupt handlers. Software interrupt handlers are attached to a software interrupt thread, just as hardware interrupt handlers are attached to a hardware interrupt thread. Multiple handlers can be attached to the same thread. Software interrupt handlers can be used to queue up less critical processing inside of hardware interrupt handlers so that the work can be done at a later time. Software interrupt threads are different from other kernel threads in that they are treated as an interrupt thread. This means that time spent executing these threads is counted as interrupt time, and that they can be run via a lightweight context switch.

The swi_add() function is used to add a new software interrupt handler to a specified interrupt event. The eventp argument is an optional pointer to a struct intr_event pointer. If this argument points to an existing event that holds a list of interrupt handlers, then this handler will be attached to that event. Otherwise a new event will be created, and if eventp is not NULL, then the pointer at that address to will be modified to point to the newly created event. The name argument is used to associate a name with a specific handler. This name is appended to the name of the software interrupt thread that this handler is attached to. The handler argument is the function that will be executed when the handler is scheduled to run. The arg parameter will be passed in as the only parameter to handler when the function is executed. The pri value specifies the priority of this interrupt handler relative to other software interrupt handlers. If an interrupt event is created, then this value is used as the vector, and the flags argument is used to specify the attributes of a handler such as INTR_MPSAFE. The cookiep argument points to a void * cookie. This cookie will be set to a value that uniquely identifies this handler, and is used to schedule the handler for execution later on.

The swi_remove() function is used to teardown an interrupt handler pointed to by the cookie argument. It detaches the interrupt handler from the associated interrupt event and frees its memory.

The swi_sched() function is used to schedule an interrupt handler and its associated thread to run. The cookie argument specifies which software interrupt handler should be scheduled to run. The flags argument specifies how and when the handler should be run and is a mask of one or more of the following flags:
SWI_DELAY Specifies that the kernel should mark the specified handler as needing to run, but the kernel should not schedule the software interrupt thread to run. Instead, handler will be executed the next time that the software interrupt thread runs after being scheduled by another event. Attaching a handler to the clock software interrupt thread and using this flag when scheduling a software interrupt handler can be used to implement the functionality performed by setdelayed() in earlier versions of FreeBSD .
  Specifies that swi_sched() is called from NMI context and should be careful about used KPIs. On platforms allowing IPI sending from NMI context it immediately wakes clk_intr_event via the IPI, otherwise it works just like SWI_DELAY.

The tty_intr_event and clk_intr_event variables contain pointers to the software interrupt handlers for the tty and clock software interrupts, respectively. tty_intr_event is used to hang tty software interrupt handlers off of the same thread. clk_intr_event is used to hang delayed handlers off of the clock software interrupt thread so that the functionality of setdelayed() can be obtained in conjunction with SWI_DELAY. The vm_ih handler cookie is used to schedule software interrupt threads to run for the VM subsystem.


The swi_add() and swi_remove() functions return zero on success and non-zero on failure.


The swi_add() function will fail if:
  The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC.
  The flags argument specifies INTR_ENTROPY.
  The eventp argument points to a hardware interrupt thread.
  Either of the name or handler arguments are NULL.
  The INTR_EXCL flag is specified and the interrupt event pointed to by eventp already has at least one handler, or the interrupt event already has an exclusive handler.

The swi_remove() function will fail if:
  A software interrupt handler pointed to by cookie is NULL.


ithread(9), taskqueue(9)


The swi_add() and swi_sched() functions first appeared in FreeBSD 5.0 . They replaced the register_swi() function which appeared in FreeBSD 3.0 and the setsoft*(), and schedsoft*() functions which date back to at least BSD 4.4 . The swi_remove() function first appeared in FreeBSD 6.1 .


Most of the global variables described in this manual page should not be global, or at the very least should not be declared in <sys/interrupt.h>.

SWI (9) July 25, 2020

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