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

Konrad Rzeszutek Wilk
2012-02-15T17:18:27Z

On Wed, Feb 15, 2012 at 01:32:04AM -0800, Steven Noonan wrote:
> 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.

Actually there is an existing self-IPI framework so that any of the smp_call_*
end up IPI-ing other CPUs and that seems to work OK.

274: 700 0 0 0 0 0 xen-percpu-ipi callfunc0
276: 15184 0 0 0 0 0 xen-percpu-ipi callfuncsingle0
279: 0 275 0 0 0 0 xen-percpu-ipi callfunc1
281: 0 8686 0 0 0 0 xen-percpu-ipi callfuncsingle1
284: 0 0 754 0 0 0 xen-percpu-ipi callfunc2
286: 0 0 4968 0 0 0 xen-percpu-ipi callfuncsingle2
289: 0 0 0 751 0 0 xen-percpu-ipi callfunc3
291: 0 0 0 19224 0 0 xen-percpu-ipi callfuncsingle3
294: 0 0 0 0 761 0 xen-percpu-ipi callfunc4
296: 0 0 0 0 21893 0 xen-percpu-ipi callfuncsingle4
299: 0 0 0 0 0 750 xen-percpu-ipi callfunc5
301: 0 0 0 0 0 10362 xen-percpu-ipi callfuncsingle5
CAL: 15886 8981 5724 19977 22657 11114 Function call interrupts

So even without Ben's patch it should have worked. I am not actually sure
why it decided to just sit there. Looking at it, the IPI on both native
and xen would end up calling: generic_smp_call_function_single_interrupt()
from their respective single-IPI interrupt handlers.

Jeremy, was there any difficulities when IPI oneself?