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

[Jan Beulich <jbeulich@suse.com>]

On 24.03.2020 16:21, Hongyan Xia wrote:
> From: Hongyan Xia <hongyxia@amazon.com>
> 
> Unfortunately, even though that commit dropped the domctl lock and
> allowed other domctl to continue, it created severe lock contention
> within domain destructions themselves. Multiple domain destructions in
> parallel now spin for the global heap lock when freeing memory and could
> spend a long time before the next hypercall continuation.

I'm not at all happy to see this reverted; instead I was hoping that
we could drop the domctl lock in further cases. If a lack of
continuations is the problem, did you try forcing them to occur more
frequently?

> 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.

Jan