tasklet
总体上，就是 在 softirq 上的封装
Linux 中断子系统: softirq 和 tasklet
从上文中分析可以看出，tasklet 是软中断的一种类型，那么两者有啥区别呢？先说结论吧：
- 软中断类型内核中都是静态分配，不支持动态分配，而 tasklet 支持动态和静态分配，也就是驱动程序中能比较方便的进行扩展；
- 软中断可以在多个 CPU 上并行运行，因此需要考虑可重入问题，而 tasklet 会绑定在某个 CPU 上运行，运行完后再解绑，不要求重入问题，当然它的性能也就会下降一些；
(绑定 CPU 确认一下)
参考 lkd chapter 8 ，不可重入指的是: Two different tasklets can run concurrently on different processors, but two of the same type of tasklet cannot run simultaneously.
The end of tasklets
https://lkml.org/lkml/2024/1/29/1604
简单分析下代码
一共使用了两个入口:
- TASKLET_SOFTIRQ
- HI_SOFTIRQ
tasklet 的使用很常见的
@[
apple_dart_map_pages+0
iommu_map+80
__iommu_dma_map+164
iommu_dma_map_page+336
dma_map_page_attrs+84
usb_hcd_map_urb_for_dma+832
xhci_map_urb_for_dma+368
usb_hcd_submit_urb+156
usb_submit_urb+452
rx_submit+328
rx_complete+736
__usb_hcd_giveback_urb+164
usb_giveback_urb_bh+208
tasklet_action_common.isra.0+236
tasklet_action+48
__do_softirq+304
____do_softirq+24
call_on_irq_stack+36
do_softirq_own_stack+36
__irq_exit_rcu+256
irq_exit_rcu+24
el1_interrupt+56
el1h_64_irq_handler+24
el1h_64_irq+104
cpuidle_enter_state+260
cpuidle_enter+64
cpuidle_idle_call+300
do_idle+168
cpu_startup_entry+64
secondary_start_kernel+224
__secondary_switched+184
]: 12214
这个是在 13900k 的环境中抓到的，所以 tasklet 没有那么不堪，还是
- 3.72% __irq_exit_rcu
- 3.70% handle_softirqs
- 3.59% tasklet_hi_action
- 3.57% bh_worker
- 3.47% process_one_work
- 3.42% usb_giveback_urb_bh
- 3.40% __usb_hcd_giveback_urb
- 3.21% btusb_intr_complete
- 1.64% btusb_recv_intr
- 0.58% queue_delayed_work_on
- 0.57% __queue_work
- 0.52% kick_pool
try_to_wake_up
- 0.57% btintel_recv_event
- hci_recv_frame
- 0.53% queue_work_on
0.53% __queue_work
- 1.49% usb_hcd_submit_urb
- 1.40% xhci_urb_enqueue
- 1.31% xhci_queue_bulk_tx
1.21% xhci_ring_ep_doorbell
此外，测试两个物理机的 iperf3 ，使用 bcc 工具 softirqs 可以观察到：
SOFTIRQ TOTAL_usecs
rcu 250
timer 478
sched 1276
hi 1579
tasklet 22394
net_tx 118328
net_rx 129335
经典例子
void __init tcp_tasklet_init(void)
{
int i;
for_each_possible_cpu(i) {
struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
INIT_LIST_HEAD(&tsq->head);
tasklet_setup(&tsq->tasklet, tcp_tasklet_func);
}
}
触发过程:
@[
tcp_wfree+0 // 在其中调用 tasklet_schedule
xmit_one.constprop.0+124
dev_hard_start_xmit+104
__dev_queue_xmit+412
neigh_hh_output+148
ip_finish_output2+940
__ip_finish_output+172
ip_finish_output+60
ip_output+112
__ip_queue_xmit+364
ip_queue_xmit+28
__tcp_transmit_skb+968
tcp_write_xmit+836
__tcp_push_pending_frames+68
tcp_push+184
tcp_sendmsg_locked+2332
tcp_sendmsg+64
inet_sendmsg+76
__sock_sendmsg+100
__sys_sendto+240
__arm64_sys_sendto+48
invoke_syscall+80
el0_svc_common.constprop.0+72
do_el0_svc+36
el0_svc+52
el0t_64_sync_handler+268
el0t_64_sync+400
]: 389
处理过程:
@[
tcp_tasklet_func+0
tasklet_action+56
handle_softirqs+300
__do_softirq+28
____do_softirq+24
call_on_irq_stack+36
do_softirq_own_stack+36
__irq_exit_rcu+316
irq_exit_rcu+24
el1_interrupt+72
el1h_64_irq_handler+24
el1h_64_irq+128
default_idle_call+56
cpuidle_idle_call+380
do_idle+244
cpu_startup_entry+64
rest_init+196
start_kernel+1104
__primary_switched+136
]: 7
就是中断处理完之后，然后继续
看看到底是如何替换为
struct workqueue_struct *system_bh_wq;
EXPORT_SYMBOL_GPL(system_bh_wq);
struct workqueue_struct *system_bh_highpri_wq;
EXPORT_SYMBOL_GPL(system_bh_highpri_wq);
哦，原来是要去彻底移除掉 tasklet 的:
/*
* TODO: Convert all tasklet users to workqueue and use softirq directly.
*
* This is currently called from tasklet[_hi]action() and thus is also called
* whenever there are tasklets to run. Let's do an early exit if there's nothing
* queued. Once conversion from tasklet is complete, the need_more_worker() test
* can be dropped.
*
* After full conversion, we'll add worker->softirq_action, directly use the
* softirq action and obtain the worker pointer from the softirq_action pointer.
*/
void workqueue_softirq_action(bool highpri)
{
struct worker_pool *pool =
&per_cpu(bh_worker_pools, smp_processor_id())[highpri];
if (need_more_worker(pool))
bh_worker(list_first_entry(&pool->workers, struct worker, node));
}
现在的调用模式为:
@[
workqueue_softirq_action+0
handle_softirqs+300
__do_softirq+28
____do_softirq+24
call_on_irq_stack+36
do_softirq_own_stack+36
__irq_exit_rcu+316
irq_exit_rcu+24
el1_interrupt+72
el1h_64_irq_handler+24
el1h_64_irq+128
selinux_inode_permission+16
inode_permission+116
link_path_walk.part.0.constprop.0+180
path_openat+132
do_filp_open+152
do_sys_openat2+140
__arm64_sys_openat+108
invoke_syscall+80
el0_svc_common.constprop.0+72
do_el0_svc+36
el0_svc+52
el0t_64_sync_handler+268
el0t_64_sync+400
]: 2
还是有点不懂的，之前 tasklet 也是在 softirq 上下文中执行的啊
tasklet
如何理解这个 tasklet_schedule ?
static void kbd_event(struct input_handle *handle, unsigned int event_type,
unsigned int event_code, int value)
{
/* We are called with interrupts disabled, just take the lock */
scoped_guard(spinlock, &kbd_event_lock) {
if (event_type == EV_MSC && event_code == MSC_RAW &&
kbd_is_hw_raw(handle->dev))
kbd_rawcode(value);
if (event_type == EV_KEY && event_code <= KEY_MAX)
kbd_keycode(event_code, value, kbd_is_hw_raw(handle->dev));
}
tasklet_schedule(&keyboard_tasklet);
do_poke_blanked_console = 1;
schedule_console_callback();
}
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