Re: [Xen-devel] [PATCH 2/2] xen: spread page scrubbing across all idle CPU

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Jun 11, 2014 at 9:13 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 11.06.14 at 04:51, <bob.liu@xxxxxxxxxx> wrote:
>> On 06/10/2014 10:12 PM, Jan Beulich wrote:
>>>>>> On 10.06.14 at 14:18, <lliubbo@xxxxxxxxx> wrote:
>>>> +    if( is_tasklet )
>>>> +        tasklet_schedule_on_cpu(&global_scrub_tasklet, cpu);
>>>
>>> So you re-schedule this tasklet immediately - while this may be
>>> acceptable inside the hypervisor, did you consider the effect this
>>> will have on the guest (normally Dom0)? Its respective vCPU won't
>>> get run _at all_ until you're done scrubbing.
>>>
>>
>> Yes, that's a problem. I don't have any better idea right now.
>>
>> What I'm trying is doing the scrubbing on current CPU as well as on all
>> idle vcpus in parallel.
>> I also considered your suggestion about doing the scrubbing in the
>> background as well as on the allocation path. But I think it's more
>> unacceptable for users to get blocked randomly for a uncertain time when
>> allocating a large mount of memory.
>> That's why I still chose the sync way that once 'xl destroy' return all
>> memory are scrubbed.
>
> But I hope you realize that in the current shape, with the shortcomings
> pointed out un-addressed, there's no way for this to go in.

Would it make more sense to do something like the following:
* Have a "clean" freelist and a "dirty" freelist
* When destroying a domain, simply move pages to the dirty freelist
* Have idle vcpus scrub the dirty freelist before going to sleep
 - ...and wake up idle vcpus to do some scrubbing when adding pages to
the dirty freelist
* In alloc_domheap_pages():
 - If there are pages on the "clean" freelist, allocate them
 - If there are no pages on the "clean" freelist but there are on the
"dirty" freelist, scrub pages from the "dirty" freelist synchronously.

 -George

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