Re: Softirq priority inversion from "softirq: reduce latencies"

From: Thomas Gleixner
Date: Mon Feb 29 2016 - 14:16:08 EST

On Mon, 29 Feb 2016, Peter Hurley wrote:
> On 02/29/2016 10:24 AM, Eric Dumazet wrote:
> >> Just to be clear
> >>
> >> if (time_before(jiffies, end) && !need_resched() &&
> >> --max_restart)
> >> goto restart;
> >>
> >> aborts softirq *even if 0ns have elapsed*, if NET_RX has woken a process.
> >
> > Sure, now remove the 1st and 2nd condition.
> Well just removing the 2nd condition has everything working fine,
> because that fixes the priority inversion.

No. It does not fix anything. It hides the shortcomings of the driver.

> However, when system resources are _not_ contended, it makes no
> sense to be forced to revert to ksoftirqd resolution, which is strictly
> intended as fallback.

No. You claim it is simply because your driver does not handle that situation

> Or flipping your argument on its head, why not just _always_ execute
> softirq in ksoftirqd?

Which is what that change effectivley does. And that makes a lot of sense,
because you get the softirq load under scheduler control and do not let the
softirq run as a context stealing entity which is completely uncontrollable by
the scheduler.

Running the softirq on return from interrupt can cause real priority