Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

[Dmitry Safonov]

On Thu, 2018-01-11 at 05:44 +0100, Frederic Weisbecker wrote:
> On Wed, Jan 10, 2018 at 08:19:49PM -0800, Linus Torvalds wrote:
> > On Wed, Jan 10, 2018 at 7:22 PM, Frederic Weisbecker
> > <frederic@xxxxxxxxxx> wrote:
> > > 
> > > Makes sense, but I think you need to keep the TASK_RUNNING check.
> > 
> > Yes, good point.
> > 
> > > So perhaps it should be:
> > > 
> > > -       return tsk && (tsk->state == TASK_RUNNING);
> > > +       return (tsk == current) && (tsk->state == TASK_RUNNING);
> > 
> > Looks good to me - definitely worth trying.
> > 
> > Maybe that weakens the thing so much that it doesn't actually help
> > the
> > UDP packet storm case?
> > 
> > And maybe it's not sufficient for the dvb issue.
> > 
> > But I think it's worth at least testing. Maybe it makes neither
> > side
> > entirely happy, but maybe it might be a good halfway point?
> 
> Yes I believe Dmitry is facing a different problem where he would
> rather
> see ksoftirqd scheduled more often to handle the queue as a deferred
> batch
> instead of having it served one by one on the tails of IRQ storms.
> (Dmitry correct me if I misunderstood).

Quite so, what I see is that ksoftirqd is rarely (close to never)
scheduled in case of UDP packet storm. That's because the up coming irq
is too late in __do_softirq(). 
So, there is no wakeup on UDP storm here:
:        pending = local_softirq_pending();
:        if (pending & mask) {
:                if (time_before(jiffies, end) && !need_resched() &&
:                    --max_restart)
:                        goto restart;
:
:                wakeup_softirqd();
:        }
(as there is yet no pending softirq). It comes a bit late to schedule
ksoftirqd and in result the next softirq is processed on the context of
the task again, not in the scheduled ksoftirqd.
That results in cpu-time starvation for the process on irq storm.

While I saw that on out-of-tree driver, I believe that on some
frequencies (lower than storm) one can observe the same on mainstream
drivers. And I *think* that I've reproduced that on mainstream with
virtio driver and package size of 1500 in VMs (thou I don't quite like
the perf testing in VMs).

So, ITOW, maybe there is a bit better way to *detect* that cpu time
spent on serving softirqs is close to storm and that userspace starts
starving? (and launch ksoftirqd in the result or balance between
deferring and serving softirq right-there).

> But your patch still seems to make sense for the case you described:
> when
> ksoftirqd is voluntarily preempted off and the current IRQ could
> handle the
> queue.