LKD: Chapter 8 Bottom Halves and Deferring Work

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　　In 2.6.x, there are 3 mechanisms for implementing a bottom half: softirqs, tasklets and work queues. Here‘s the comparison:

Softirqs:

　　Softirqs are statically allocated at compile time. It is represented by the softirq_action structure, which is defined in <linux/interrupt.h>:

struct softirq_action {
    void (*action)(struct softirq_action *);
}

 　　A 32-entry array of this structure is declared in kernel/softirq.c:

static struct softirq_action softirq_vec[NR_SOFTIRQS];

　　But in the current kernel, only nine exist:(as we will discuss later, tasklets are built off softirqs)

　　The prototype of a softirq handler looks like

void softirq_handler(struct softirq_action *)

　　A softirq never preempts another softirq. The only event that can preempt a softirq is an interrupt handler.

Executing Softirqs:

　　A registered softirq must be marked before it will execute. This is called raising the softirq.

　　Softirq execution occurs in __do_softirq(), which is invoked by do_softirq(). If there are pending softirqs, __do_softirq() loops over each one, invoking its handler. Let‘s look at a simplified variant of the important part of __do_softirq():

u32 pending;

pending = local_softirq_pending();
if (pending) {
    struct softirq_action *h;

    /* reset the pending bitmask */
    set_softirq_pending(0);

    h = softirq_vec;
    do {
        if (pending & 1)
            h->action(h);
        h++;
        pending >>= 1;
    } while (pending);
}

Using Softirqs:

　　Softirqs are reserved for the most timing-critical and important bottom-half processing on the system.  Currently, only two subsystems - networking and block devices - directly use softirqs.

　　Registering Your Handler:

　　The softirq handler is registered at run-time via open_softirq():

/* in net/core/dev.c */
/* two parameters: the sfotirq‘s index and its handler function */
open_softirq(NET_TX_SOFTIRQ, net_tx_action);

　　Raising Your Softirq:

　　To mark it pending, call raise_softirq():　　　　

raise_softirq(NET_TX_SOFTIRQ);

　　Then it is run at the next invocation of do_softirq().

LKD: Chapter 8 Bottom Halves and Deferring Work

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原文地址：http://www.cnblogs.com/justforfun12/p/5071664.html