Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv guest under Xen with single vcpu

[Sander Eikelenboom]

On 2015-12-14 20:48, Eric Shelton wrote:
> Please note that the same issue appears to have been introduced in the
> recent 4.2.7 kernel.  It perhaps has to do
> with b4ff8389ed14b849354b59ce9b360bdefcdbf99c having a matching
> commit e8d097151d309eb71f750bbf34e6a7ef6256da7e in linux-stable.git.  
> The
> below patch to arch/x86/kernel/rtc.c was also effective for 4.2.7.
>
> Eric

Hi Eric,

Yeah it's unfortunate the patch patching the other patches destined for 
stable didn't make it in time for stable :(.
Any how the chosen solution wasn't ideal so there now is a V2 patch by 
Boris. It hasn't been picked up yet,
but hopefully will be anytime soon (for the patch see 
http://lkml.iu.edu/hypermail/linux/kernel/1512.1/03504.html)

--
Sander

> On 2015-12-02 18:30, Sander Eikelenboom wrote:
> > On 2015-12-02 15:55, David Vrabel wrote:
> > > On 28/11/15 15:47, Sander Eikelenboom wrote:
> > >> genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000
> > >> (rtc0)
> > >
> > > We shouldn't register an rtc_cmos device because its legacy irq
> > > conflicts with the irq needed for hvc0.  For a multi VCPU guest irq 8
> > > is
> > > in use for the pv spinlocks and this gets requested first, preventing
> > > the rtc device from probing.
> > >
> > > Does this patch fix it for you?
> > >
> > > David
> >
> > It does, thanks.
> >
> > Reported-and-tested-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> >
> > --
> > Sander
> >
> > > 8<--------------------
> > > x86: rtc_cmos platform device requires legacy irqs
> > >
> > > Adding the rtc platform device when there are no legacy irqs (no
> > > legacy PIC) causes a conflict with other devices that end up using the
> > > same irq number.
> > >
> > > In a single VCPU PV guest we should have:
> > >
> > > /proc/interrupts:
> > >            CPU0
> > >   0:       4934  xen-percpu-virq      timer0
> > >   1:          0  xen-percpu-ipi       spinlock0
> > >   2:          0  xen-percpu-ipi       resched0
> > >   3:          0  xen-percpu-ipi       callfunc0
> > >   4:          0  xen-percpu-virq      debug0
> > >   5:          0  xen-percpu-ipi       callfuncsingle0
> > >   6:          0  xen-percpu-ipi       irqwork0
> > >   7:        321   xen-dyn-event     xenbus
> > >   8:         90   xen-dyn-event     hvc_console
> > >   ...
> > >
> > > But hvc_console cannot get its interrupt because it is already in use
> > > by rtc0 and the console does not work.
> > >
> > >   genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000
> > > (rtc0)
> > >
> > > The rtc_cmos device requires a particular legacy irq so don't add it
> > > if there are no legacy irqs.
> > >
> > > Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kernel/rtc.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/arch/x86/kernel/rtc.c b/arch/x86/kernel/rtc.c
> > > index cd96852..07c70f1 100644
> > > --- a/arch/x86/kernel/rtc.c
> > > +++ b/arch/x86/kernel/rtc.c
> > > @@ -14,6 +14,7 @@
> > >  #include <asm/time.h>
> > >  #include <asm/intel-mid.h>
> > >  #include <asm/rtc.h>
> > > +#include <asm/i8259.h>
> > >
> > >  #ifdef CONFIG_X86_32
> > >  /*
> > > @@ -200,6 +201,10 @@ static __init int add_rtc_cmos(void)
> > >       }
> > >  #endif
> > >
> > > +     /* RTC uses legacy IRQs. */
> > > +     if (!nr_legacy_irqs())
> > > +             return -ENODEV;
> > > +
> > >       platform_device_register(&rtc_device);
> > >       dev_info(&rtc_device.dev,
> > >                "registered platform RTC device (no PNP device
> found)\n");
--
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