Re: [PATCH] Revert "domctl: improve locking during domain destruction"

[Julien Grall <julien@xen.org>]

Hi Jim,

On 26/03/2020 16:55, Jim Fehlig wrote:
> On 3/25/20 1:11 AM, Jan Beulich wrote:
>> On 24.03.2020 19:39, Julien Grall wrote:
>>> On 24/03/2020 16:13, Jan Beulich wrote:
>>>> On 24.03.2020 16:21, Hongyan Xia wrote:
>>>>> From: Hongyan Xia <hongyxia@amazon.com>
>>>>> In contrast,
>>>>> after dropping that commit, parallel domain destructions will just 
>>>>> fail
>>>>> to take the domctl lock, creating a hypercall continuation and backing
>>>>> off immediately, allowing the thread that holds the lock to destroy a
>>>>> domain much more quickly and allowing backed-off threads to process
>>>>> events and irqs.
>>>>>
>>>>> On a 144-core server with 4TiB of memory, destroying 32 guests (each
>>>>> with 4 vcpus and 122GiB memory) simultaneously takes:
>>>>>
>>>>> before the revert: 29 minutes
>>>>> after the revert: 6 minutes
>>>>
>>>> This wants comparing against numbers demonstrating the bad effects of
>>>> the global domctl lock. Iirc they were quite a bit higher than 6 min,
>>>> perhaps depending on guest properties.
>>>
>>> Your original commit message doesn't contain any clue in which
>>> cases the domctl lock was an issue. So please provide information
>>> on the setups you think it will make it worse.
>>
>> I did never observe the issue myself - let's see whether one of the SUSE
>> people possibly involved in this back then recall (or have further
>> pointers; Jim, Charles?), or whether any of the (partly former) Citrix
>> folks do. My vague recollection is that the issue was the tool stack as
>> a whole stalling for far too long in particular when destroying very
>> large guests.
> 
> I too only have a vague memory of the issue but do recall shutting down 
> large guests (e.g. 500GB) taking a long time and blocking other 
> toolstack operations. I haven't checked on the behavior in quite some 
> time though.

It might be worth checking how toolstack operations (such as domain 
creating) is affected by the revert. @Hongyan would you be able to test it?

> 
>> One important aspect not discussed in the commit message
>> at all is that holding the domctl lock block basically _all_ tool stack
>> operations (including e.g. creation of new guests), whereas the new
>> issue attempted to be addressed is limited to just domain cleanup.
> 
> I more vaguely recall shutting down the host taking a *long* time when 
> dom0 had large amounts of memory, e.g. when it had all host memory (no 
> dom0_mem= setting and autoballooning enabled).

AFAIK, we never relinquish memory from dom0. So I am not sure how a 
large amount of memory in Dom0 would affect the host shutting down.

Cheers,

-- 
Julien Grall