Re: [Xen-devel] [PATCH] Avoid race when moving cpu between cpupools

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 02/25/11 15:25, Andre Przywara wrote:
> George Dunlap wrote:
> > Looks good -- thanks Juergen.
> >
> > Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> >
> > -George
> >
> > On Thu, Feb 24, 2011 at 2:08 PM, Andre Przywara
> > <andre.przywara@xxxxxxx> wrote:
> > > Juergen Gross wrote:
> > > > Moving cpus between cpupools is done under the schedule lock of the
> > > > moved
> > > > cpu.
> > > > When checking a cpu being member of a cpupool this must be done with
> > > > the
> > > > lock
> > > > of that cpu being held.
> > > I have reviewed and tested the patch. It fixes my problem. My script has
> > > been running for several hundred iterations without any Xen crash,
> > > whereas
> > > without the patch the hypervisor crashed mostly at the second iteration.
> Juergen,
>
> can you rule out that this code will be triggered on two CPUs trying to
> switch to each other? As Stephan pointed out: the code looks like as
> this could trigger a possible dead-lock condition, where:
> 1) CPU A grabs lock (a) while CPU B grabs lock (b)
> 2) CPU A tries to grab (b) and CPU B tries to grab (a)
> 3) both fail and loop to 1)
Good point. Not quite a dead-lock, but a possible live-lock :-)

> A possible fix would be to introduce some ordering for the locks (just
> the pointer address) and let the "bigger" pointer yield to the "smaller"
> one.
Done this and sent a patch.

> I am not sure if this is really necessary, but I now see strange
> hangs after running the script for a while (30min to 1hr).
> Sometimes Dom0 hangs for a while, loosing interrupts (sda or eth0) or
> getting spurious ones, on two occasions the machine totally locked up.
>
> I am not 100% sure whether this is CPUpools related, but I put some load
> on Dom0 (without messing with CPUpools) for the whole night and it ran
> fine.
Did you try to do this with all Dom0-vcpus pinned to 6 physical cpus?
I had the same problems when using only few physical cpus for many vcpus.
And I'm pretty sure this was NOT the possible live-lock, as it happened
already without this change when I tried to reproduce your problem.

> Sorry for this :-(
> I will try to further isolate this.
>
> Anyway, it works much better with the fix than without and I will try to
> trigger this with the "reduce number of Dom0 vCPUs" patch.
Thanks, Juergen

--
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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