Re: [Xen-devel] issues with PLE and/or scheduler.

["Shan, Haitao" <haitao.shan@xxxxxxxxx>]

We have reproduced your problem locally and are looking into this issue. It
seems "PLE with timer mode 2" will trigger the issue. We can post our
findings as soon as possible.

Shan Haitao

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Konrad Rzeszutek Wilk
> Sent: Wednesday, December 21, 2011 4:42 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx;
> George.Dunlap@xxxxxxxxxxxxx; keir@xxxxxxx; andrew.thomas@xxxxxxxxxx
> Subject: Re: [Xen-devel] issues with PLE and/or scheduler.
> 
> On Tue, Dec 20, 2011 at 04:41:07PM -0400, Konrad Rzeszutek Wilk wrote:
> > Hey folks,
> >
> > I am sending this on behalf of Andrew since our internal email system
> > is dropping all xen-devel mailing lists :-(
> 
> <hits his head> And I forgot to CC andrew on it. Added here.
> >
> > Anyhow:
> >
> > This is with xen-4.1-testing cs 23201:1c89f7d29fbb and using the
> > default "credit" scheduler.
> >
> > I've run into an interesting issue with HVM guests which make use of
> > Pause Loop Exiting (ie. on westmere systems; and also on romley
> > systems):  after yielding the cpu, guests don't seem to receive timer
> > interrupts correctly..
> >
> > Some background: for historical reasons (ie old templates) we boot
> > OL/RHEL guests with the following settings:
> >
> > kernel parameters: clock=pit nohpet nopmtimer
> > vm.cfg: timer_mode = 2
> >
> > With PLE enabled, 2.6.32 guests will crash early on with:
> >  ..MP-BIOS bug: 8254 timer not connected to IO-APIC  # a few lines
> > omitted..
> >  Kernel panic - not syncing: IO-APIC + timer doesn't work!  Boot with
> > apic=debug
> >
> > While 2.6.18-238 (ie OL/RHEL5u6) will fail to find the timer, but
> > continue and lock up in the serial line initialization.
> >
> >  ..MP-BIOS bug: 8254 timer not connected to IO-APIC  # continues until
> > lock up here:
> >  Serial: 8250/16550 driver $Revision: 1.90 $ 4 ports, IRQ sharing
> > enabled
> >
> > Instrumenting the 2.6.32 code (ie timer_irq_works()) shows that
> > jiffies isn't advancing (or only 1 or 2 ticks are being received,
> > which is insufficient for "working"). This is on a "quiet" system with
no
> other activity.
> > So, even though the guest has voluntarily yielded the cpu (through
> > PLE), I would still expect it to receive every clock tick (even with
> > timer_mode=2) as there is no other work to do on the system.
> >
> > Disabling PLE allows both 2.6.18 and 2.6.32 guests to boot.. [As an
> > aside, so does setting ple_gap to 41 (ie prior to 21355:727ccaaa6cce)
> > -- the perf counters show no exits happening, so this is equivalent to
> > disabling PLE.]
> >
> > I'm hoping someone who knows the scheduler well will be able to
> > quickly decide whether this is a bug or a feature...
> >
> > Andrew
> 
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