Re: [PATCH][RT] netpoll: Always take poll_lock when doing polling

[Steven Rostedt]

On Fri, 10 Jun 2016 17:57:17 +0200
Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:

> * Steven Rostedt | 2016-06-04 07:11:31 [-0400]:
> 
> >From: Steven Rostedt <srostedt@xxxxxxxxxx>
> >Date: Tue, 5 Jan 2016 14:53:09 -0500
> >Subject: [PATCH] softirq: Perform softirqs in local_bh_enable() for a limited
> > amount of time
> >
> >To prevent starvation of tasks like ksoftirqd, if the task that is
> >processing its softirqs takes more than 2 jiffies to do so, and the
> >softirqs are constantly being re-added, then defer the processing to
> >ksoftirqd.  
> 
> I'm not sure about this. Alison didn't scream "yes it solves my problem"
> and I am not sure what it is.
> It is true that in RT we don't have such a limit like in !RT. You would
> need to use __raise_softirq_irqoff_ksoft() instead the normal or +
> wakeup() since you may have timers pending and those need to go to the
> "other" ksoftirqd.
> But then I don't see much change. ksoftirqd runs now at SCHED_OTHER so
> it will end up on the CPU right away unless there other tasks that need
> the CPU. So the scheduler will balance it the same way. The only change
> will be that softirqs which are processed in context of any application
> for more than two jiffies will be moved to ksoftirqd. This could be a
> win.

We actually triggered a starvation due to this. I was just seeing if
Alison hit the same issue we did in our tests.

-- Steve