Re: [Xen-devel] [PATCH] SEDF: avoid gathering vCPU-s on pCPU0

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Fri, Mar 1, 2013 at 3:35 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> The introduction of vcpu_force_reschedule() in 14320:215b799fa181 was
> incompatible with the SEDF scheduler: Any vCPU using
> VCPUOP_stop_periodic_timer (e.g. any vCPU of half way modern PV Linux
> guests) ends up on pCPU0 after that call. Obviously, running all PV
> guests' (and namely Dom0's) vCPU-s on pCPU0 causes problems for those
> guests rather sooner than later.
>
> So the main thing that was clearly wrong (and bogus from the beginning)
> was the use of cpumask_first() in sedf_pick_cpu(). It is being replaced
> by a construct that prefers to put back the vCPU on the pCPU that it
> got launched on.
>
> However, there's one more glitch: When reducing the affinity of a vCPU
> temporarily, and then widening it again to a set that includes the pCPU
> that the vCPU was last running on, the generic scheduler code would not
> force a migration of that vCPU, and hence it would forever stay on the
> pCPU it last ran on. Since that can again create a load imbalance, the
> SEDF scheduler wants a migration to happen regardless of it being
> apparently unnecessary.

I'm not quite understanding what this is about -- why is this
necessary for sedf but not for the other schedulers?

 -George

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