[Xen-devel] issue with PLE and/or scheduler

[andrew thomas <andrew.thomas@xxxxxxxxxx>]

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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