Re: backport request for Xen 4.2 and 4.1 of cs 26287:127c2c47d440

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

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Am 25.01.2013 10:49, schrieb Juergen Gross:
> Hi,
>
> I request backport of changeset 26287:127c2c47d440 to Xen 4.2 and Xen 4.1.
>
> Without this change we experience Dom0 lockups when cpupool0 has all
> cpus but
> one on a single socket and xen ticketlocks are disabled in Dom0.
>
> Some background information:
>
> Disabling ticketlocks in Dom0 was done to avoid lock starvation on certain
> workloads with one lock in Dom0 taken at a higher frequency as ticketlock
> processing would allow (the poll hypercall was much more expensive than
> waiting
> for the lock via spinning). In our configuration Dom0 is the only domain in
> cpupool0 so using xen ticketlocks shouldn't be an advantage.
>
> The Dom0 lockup happened when one vcpu was holding the xtime_lock while all
> other vcpus tried to grab it and none of the vcpus was running on the
> single
> core on the other socket. At some time all vcpus were spinning and the one
> core was idle without being considered for vcpu migration (this should be
> addressed by the changeset above). As none of the vcpus would give up
> control
> voluntarily the complete time slice is used with one vcpu of Dom0 having no
> physical processor to run on. During that time the next timer interrupt
> will
> be pending leading to another request of the xtime_lock which will
> trigger the
> same loop over and over again.
>
>
> Juergen
>


-- 
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@ts.fujitsu.com
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