Re: bisected: 'perf top' causing soft lockups under Xen

Konrad Rzeszutek Wilk
2012-02-15T16:21:17Z

On Wed, Feb 15, 2012 at 10:25:44AM +0100, Peter Zijlstra wrote:
> On Wed, 2012-02-15 at 00:57 -0800, Steven Noonan wrote:
> > It seems to me that there are two options for fixing this, but I'm
> > probably lacking the necessary context (or experience with Xen). Either:
> >
> > - The patch provided by Ben needs to have additional work to specially
> > handle IRQ_WORK_VECTOR, since it seems to be a special case where
> > there's no event channel attached for it. Perhaps adding an event
> > channel for this is the fix? Seems high-overhead, but I lack a good
> > understanding of how interrupts are handled in Xen.
>
> So that's a self-IPI, is Xen failing to implement this?

It does have self-IPIs.
>
> > or
> >
> > - Perf needs to be "enlightened" about Xen and avoid sending an IPI in
> > the first place.
>
> Uhm, no. If anything Xen should simply not implement
> arch_irq_work_raise(). The callbacks are then ran from the timer
> interrupt.

Looks like that wouldn't be too difficult - meaning implement a similar
form of IRQ_WORKER that would call

inc_irq_stat(apic_irq_work_irqs);
irq_work_run();

.. along with the rest of the stuff from Ben's patch. Let me see if I can
prep a patch.