Re: [Xen-devel] [PATCH 1/3] xen: sched: introduce the 'null' semi-static scheduler

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Mon, Mar 27, 2017 at 11:31 AM, George Dunlap
<george.dunlap@xxxxxxxxxx> wrote:
> On 17/03/17 18:42, Dario Faggioli wrote:
>> In cases where one is absolutely sure that there will be
>> less vCPUs than pCPUs, having to pay the cose, mostly in
>> terms of overhead, of an advanced scheduler may be not
>> desirable.
>>
>> The simple scheduler implemented here could be a solution.
>> Here how it works:
>>  - each vCPU is statically assigned to a pCPU;
>>  - if there are pCPUs without any vCPU assigned, they
>>    stay idle (as in, the run their idle vCPU);
>>  - if there are vCPUs which are not assigned to any
>>    pCPU (e.g., because there are more vCPUs than pCPUs)
>>    they *don't* run, until they get assigned;
>>  - if a vCPU assigned to a pCPU goes away, one of the
>>    waiting to be assigned vCPU, if any, gets assigned
>>    to the pCPU and can run there.
>
> Hmm -- I'm not sure about this 'waitqueue' thing.  If you have a
> multi-vcpu VM and one vcpu hangs, what normally happens is that the rest
> of the VM ends up wedging itself in an unpredictable way, and if there's
> a watchdog timer or sanity check of any sort then it will hit a
> bugcheck.  As implemented, any number of mundane operations may cause
> such a situation if you have one less pcpu or one more vcpu than you
> thought.  This seems like a fairly "sharp edge" to have in the interface.
>
> Would it be possible instead to have domain assignment, vcpu-add /
> remove, pcpu remove, &c just fail (perhaps with -ENOSPC and/or -EBUSY)
> if we ever reach a situation where |vcpus| > |pcpus|?
>
> Or, to fail as many operations *as possible* which would bring us to
> that state, use the `waitqueue` idea as a backup for situations where we
> can't really avoid it?

I suppose one reason is that it looks like a lot of the operations
can't really fail -- insert_vcpu and deinit_pdata both return void,
and the scheduler isn't even directly involved in setting the hard
affinity, so doesn't get a chance to object that with the new hard
affinity there is nowhere to run the vcpu.

I don't want to wait to re-write the interfaces to get this scheduler
in, so I suppose the waitqueue thing will have to do for now. :-)

 -George

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