On Mon, 2013-06-10 at 13:36 -0400, Alan Stern wrote: > > I don't think so, and obviously softirq allows IRQs to be served quickly, > > otherwise why is softirq and tasklet introduced? Because of history. In the old days there were top halves (hard irq) and bottom halves (like a softirq). The thing about a bottom half was, no two could run at the same time. That is, if one CPU was running a bottom half, another CPU would have to wait for that one to complete before it could run any bottom half. The killer here is that it didn't matter if it was the same bottom half or not! Microsoft/Mindcraft was kind enough to point out this inefficiency with some benchmarks showing how horrible Linux ran on a 4 way box. Thanks to them, softirqs and tasklets were born :-) A softirq does the rest of the interrupt work with interrupts enabled. A tasklet is run from softirq context, but the difference between a softirq and tasklet is that the same tasklet can not run on two different CPUs at the same time. It acts similar to a task, as a task can not run on two different CPUs at the same time either, hence the name "tasklet". But tasklets are better than bottom halves because it allows for different tasklets to run on different CPUs. > and why so many drivers > > are using tasklet/softirq? Because it's easy to set up and device driver authors don't know any better ;-). Note, a lot of drivers are now using work queues today, which run as a task. Yes, there's a little more overhead with a task than for a softirq, but the problem with softirqs and tasklets is that they can't be preempted, and they are more important than all tasks on the system. If you have a task that is critical, it must yield for your softirq. Almost! That is, even if you have a softirq, it may not run in irq context at all. If you get too many softirqs at a time (one comes while another one is running), it will defer the processing to the ksoftirq thread. This not only runs as a task, but also runs as SCHED_OTHER, and must yield to other tasks like everyone else too. Thus, adding it as a softirq does not guarantee that it will be processed quickly. It just means that most of the time it will prevent anything else from happening while your "most important handler in the world" is running. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
