[RFC PATCH 0/4] USB: HCD/EHCI: giveback of URB in tasklet context

[Steven Rostedt]

On Mon, 2013-06-10 at 16:47 -0400, Alan Stern wrote:
> On Mon, 10 Jun 2013, Steven Rostedt wrote:
> 
> > >  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.
> 
> >From this, it sounds like you generally advise using threaded interrupt 
> handlers rather than tasklets/softirqs.

Yes, there's plenty of benefits for them, and I highly doubt that any
USB device would even notice the difference between a softirq and a
thread for response time latencies.

-- Steve