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

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Date:	Mon, 14 Dec 2015 22:07:30 +0100
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Eric Shelton <eshelton@...ox.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org,
	knockknock@...il.com
Subject: Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv
 guest under Xen with single vcpu

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@...elenboom.it>
>> 
>> --
>> 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@...rix.com>
>> > ---
>> >  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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