include/linux/interrupt.h
/* PLEASE, avoid to allocate new softirqs, if you need not _really_ high
frequency threaded job scheduling. For almost all the purposes
tasklets are more than enough. F.e. all serial device BHs et
al. should be converted to tasklets, not to softirqs.
*/
enum
{
HI_SOFTIRQ=0, /* High Priority */
TIMER_SOFTIRQ,
NET_TX_SOFTIRQ,
NET_RX_SOFTIRQ,
BLOCK_SOFTIRQ,
BLOCK_IOPOLL_SOFTIRQ,
TASKLET_SOFTIRQ,
SCHED_SOFTIRQ,
HRTIMER_SOFTIRQ,
RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */
NR_SOFTIRQS
};
The key differences between softirq and tasklet are:
Allocation
- Softirqs are statically allocated at compile-time. Unlike tasklets, you cannot dynamically register and destroy softirqs.
- Tasklets can be statically allocated using
DECLARE_TASKLET(name, func, data) or can also be allocated dynamically and initialized at runtime using tasklet_init(name, func, data)
Concurrency
- Softirqs can run concurrently on several CPUs, even if they are of the same type because softirqs are
reentrant functions and must explicitly protect their data structures with spinlocks.
- Tasklets are
non-reentrant and tasklets of the same type are always serialized: in other words, the same type of tasklet cannot be executed by two CPUs at the same time. However, tasklets of different types can be executed concurrently on several CPUs.
Processing
- Softirqs are activated by means of the
raise_softirq(). The pending softirqs are processed by do_softirq() and ksoftirqd kernel thread after being enabled by local_bh_enable() or by spin_unlock_bh()
- Tasklets are a bottom-half mechanism built on top of softirqs i.e. tasklets are represented by two softirqs:
HI_SOFTIRQ and TASKLET_SOFTIRQ. Tasklets are actually run from a softirq. The only real difference in these types is that the HI_SOFTIRQ based tasklets run prior to the TASKLET_SOFTIRQ tasklets. So, tasklet_schedule() basically calls raise_softirq(TASKLET_SOFTIRQ)
- Note that softirqs (and hence tasklets and timers) are run on return from hardware interrupts, or on return from a system call. Also as soon as the thread that raised the softirq ends, that single softirq (and on other) is run to minimize
softirq latency.
