Re: [Xen-devel] [PATCH] SEDF: avoid gathering vCPU-s on pCPU0 [and 1 more messages]

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Jan Beulich writes ("Re: [Xen-devel] [PATCH] SEDF: avoid gathering vCPU-s on 
pCPU0"):
> On 02.03.13 at 04:44, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> > Is this change at all related to the long standing failure to boot with
> > sedf in the test harnesss?
> 
> Yes, very likely. It was looking at the logs (where all Dom0 vCPU-s
> were similarly running on pCPU0) plus immediately seeing a hang
> when trying it out that prompted me to finally look into the issue.

Good, but...

Jan Beulich writes ("Re: [Xen-devel] [PATCH] SEDF: avoid gathering vCPU-s on 
pCPU0"):
...
> So when a vCPU narrows its affinity (in the extreme to a mask
> with just one bit set) and then widens it again, it would
> nevertheless remain on the pCPU it was formerly restricted to.
> When (perhaps much later) a second and then a third vCPU
> do the same, they may all end up running on the same pCPU,
> i.e. we'd get back to the problem addressed by the first half of
> the fix here.

I don't understand why having every VCPU in the whole system running
on a single PCPU would result in a complete hang.  Surely unless we
have many VCPUs all piling on spinlocks there will not be that many
runnable VCPUs ?

Or is this symptom due to the lack of PV spinlocks ?

Ian.

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