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

[Steven Noonan <steven@uplinklabs.net>]

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?

Yes.

Ben's patch implements it, but it explodes (NULL pointer dereference)
when it can't find an event channel for IRQ_WORK_VECTOR.

> 
> > 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.

Sorry, wild guess. I'm a kernel newbie. :)