Re: [Xen-devel] Scheduler regression in 4.7

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2016-08-11 at 14:39 +0100, Andrew Cooper wrote:
> On 11/08/16 14:24, George Dunlap wrote:
> > On 11/08/16 12:35, Andrew Cooper wrote:
> > > The actual cause is _csched_cpu_pick() falling over LIST_POISON,
> > > which
> > > happened to occur at the same time as a domain was shutting
> > > down.  The
> > > instruction in question is `mov 0x10(%rax),%rax` which looks like
> > > reverse list traversal.
>
Thanks for the report.

> > Could you use line2addr or objdump -dl to get a better idea where
> > the
> > #GP is happening?
> addr2line -e xen-syms-4.7.0-xs127493 ffff82d08012944f
> /obj/RPM_BUILD_DIRECTORY/xen-4.7.0/xen/common/sched_credit.c:775
> (discriminator 1)
> 
> It will be IS_RUNQ_IDLE() which is the problem.
> 
Ok, that does one step of list traversing (the runq). What I didn't
understand from your report is what crashed when.

IS_RUNQ_IDLE() has been introduced a while back and anything like that
has been ever caught so far. George's patch makes _csched_cpu_pick() be
called during insert_vcpu()-->csched_vcpu_insert() which, in 4.7, is
called:
 1) during domain (well, vcpu) creation,
 2) when domain is moved among cpupools

AFAICR, during domain destruction we basically move the domain to
cpupool0, and without a patch that I sent recently, that is always done
as a full fledged cpupool movement, even if the domain is _already_ in
cpupool0. So, even if you are not using cpupools, and since you mention
domain shutdown we probably are looking at 2).

But this is what I'm not sure I got well... Do you have enough info to
tell precisely when the crash manifests? Is it indeed during a domain
shutdown, or was it during a domain creation (sched_init_vcpu() is in
the stack trace... although I've read it's a non-debug one)? And is it
a 'regular' domain or dom0 that is shutting down/coming up?

The idea behind IS_RUNQ_IDLE() is that we need to know whether there is
someone in the runq of a cpu or not, to correctly initialize --and
hence avoid biasing-- some load balancing calculations. I've never
liked the idea (leave it alone the code), but it's necessary (or, at
least, I don't see a sensible alternative).

The questions I'm asking above have the aim of figuring out what the
status of the runq could be, and why adding a call to csched_cpu_pick()
from insert_vcpu() is making things explode...

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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