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

[Hongyan Xia <hx242@xxxxxxx>]

From: Hongyan Xia <hongyxia@xxxxxxxxxx>

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. 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 is timed between the first page and the very last page of all 32
guests is released back to the heap.

This reverts commit 228ab9992ffb1d8f9d2475f2581e68b2913acb88.

Signed-off-by: Hongyan Xia <hongyxia@xxxxxxxxxx>
---
 xen/common/domain.c | 11 +----------
 xen/common/domctl.c |  5 +----
 2 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index b4eb476a9c..7b02f5ead7 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -698,20 +698,11 @@ int domain_kill(struct domain *d)
     if ( d == current->domain )
         return -EINVAL;
 
-    /* Protected by d->domain_lock. */
+    /* Protected by domctl_lock. */
     switch ( d->is_dying )
     {
     case DOMDYING_alive:
-        domain_unlock(d);
         domain_pause(d);
-        domain_lock(d);
-        /*
-         * With the domain lock dropped, d->is_dying may have changed. Call
-         * ourselves recursively if so, which is safe as then we won't come
-         * back here.
-         */
-        if ( d->is_dying != DOMDYING_alive )
-            return domain_kill(d);
         d->is_dying = DOMDYING_dying;
         argo_destroy(d);
         evtchn_destroy(d);
diff --git a/xen/common/domctl.c b/xen/common/domctl.c
index a69b3b59a8..e010079203 100644
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -571,14 +571,11 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
u_domctl)
         break;
 
     case XEN_DOMCTL_destroydomain:
-        domctl_lock_release();
-        domain_lock(d);
         ret = domain_kill(d);
-        domain_unlock(d);
         if ( ret == -ERESTART )
             ret = hypercall_create_continuation(
                 __HYPERVISOR_domctl, "h", u_domctl);
-        goto domctl_out_unlock_domonly;
+        break;
 
     case XEN_DOMCTL_setnodeaffinity:
     {
-- 
2.17.1