Date: Wed, 15 Feb 2012 11:17:41 -0500
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Noonan <steven@uplinklabs.net>, Ben Guthro <ben@guthro.net>,
        linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
        Ingo Molnar <mingo@elte.hu>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: bisected: 'perf top' causing soft lockups under Xen
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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.

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