Re: [Xen-devel] [PATCHv1 6/8] xen/balloon: only hotplug additional memory if required

[David Vrabel <david.vrabel@citrix.com>]

On 26/06/15 13:46, Daniel Kiper wrote:
> On Fri, Jun 26, 2015 at 09:56:31AM +0100, David Vrabel wrote:
>> On 25/06/15 22:18, Daniel Kiper wrote:
>>> On Thu, Jun 25, 2015 at 06:11:01PM +0100, David Vrabel wrote:
>>>> Now that we track the total number of pages (included hotplugged
>>>> regions), it is easy to determine if more memory needs to be
>>>> hotplugged.
>>>>
>>>> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
>>>> ---
>>>>  drivers/xen/balloon.c |   16 +++++++++++++---
>>>>  1 file changed, 13 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>>>> index 960ac79..dd41da8 100644
>>>> --- a/drivers/xen/balloon.c
>>>> +++ b/drivers/xen/balloon.c
>>>> @@ -241,12 +241,22 @@ static void release_memory_resource(struct resource *resource)
>>>>   * bit set). Real size of added memory is established at page onlining stage.
>>>>   */
>>>>
>>>> -static enum bp_state reserve_additional_memory(long credit)
>>>> +static enum bp_state reserve_additional_memory(void)
>>>>  {
>>>> +	long credit;
>>>>  	struct resource *resource;
>>>>  	int nid, rc;
>>>>  	unsigned long balloon_hotplug;
>>>>
>>>> +	credit = balloon_stats.target_pages - balloon_stats.total_pages;
>>>> +
>>>> +	/*
>>>> +	 * Already hotplugged enough pages?  Wait for them to be
>>>> +	 * onlined.
>>>> +	 */
>>>
>>> Comment is wrong or at least misleading. Both values does not depend on onlining.
>>
>> If we get here and credit <=0 then the balloon is empty and we have
> 
> Right.
> 
>> already hotplugged enough sections to reach target.  We need to wait for
> 
> OK.
> 
>> userspace to online the sections that already exist.
> 
> This is not true. You do not need to online sections to reserve new
> memory region. Onlining does not change balloon_stats.target_pages
> nor balloon_stats.total_pages. You must increase balloon_stats.target_pages
> above balloon_stats.total_pages to reserve new memory region. And
> balloon_stats.target_pages increase is not related to onlining.

We don't want to keep adding sections if onlining the existing ones
would be sufficient to reach the target.

David