Difference between SoftIRQs and Tasklets

Question

While studying Linux interrupt handling I found that Tasklets and SoftIRQs are two different methods of performing "bottom half" (lesser priority work). I understand this (quite genuine need).

Difference being, SoftIRQs are re-entarant while a Tasklet is NOT. That same SoftIRQ can run on different CPUs while this is NOT the case with Tasklets.

Though I understand this from surface but I fail in understanding the requirements of the two features. In what case(s) we may use these facilities ? How to recognize that I should use Tasklets now and SoftIRQs then.

Also what do we mean by Tasklets are made upon SoftIRQs ? In one of the books I read in LKML there were debates upon removing Tasklets. I got completely confused why one would bring in such a feature ? Some shortsightedness (No offense meant) ?

Any pointers on this will help a lot.

Answer 1

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.