[Xen-devel] Strange vcpu count on forced domain crash

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

This is likely not a bug, but I know there's been some work in
hotplug CPU support lately, so I thought I'd report this on the
chance that it is a bug:

For benchmarking purposes (to stop a guest immediately and
leave it in a crashed state), I use:

# echo c > /proc/sysrq-trigger

in the (PV) guest.

I've noticed that when I do this on a vcpus==2 PV guest,
the "xm list" then shows it as vcpus==1, while xentop
continues to report it as vcpus==2.

When this happens, I often get a flurry of Xen messages from
memory.c and/or page_alloc.c, of the type:

(XEN) page_alloc.c:1083:d6 Over-allocation for domain 6: 131073 > 131072
(XEN) memory.c:132:d6 Could not allocate order=0 extent: id=6 memflags=0 (0 of 5
12)

though at other times I've also seen a bunch of:

(XEN) mm.c:3798:d3 Bad donate 0000000000067a90: ed=ffff830069140000(3), sd=00000
00000000000, caf=8000000000000001, taf=0000000000000000

and once a crash with:

(XEN) mm.c:2363:d0 Bad type (saw 7400000001000000 != exp 1000000000000000) for m
fn 7af88 (pfn c78)
(XEN) mm.c:867:d0 Attempt to create linear p.t. with write perms
(XEN) mm.c:1329:d0 Failure in alloc_l2_table: entry 64
(XEN) mm.c:2116:d0 Error while validating mfn 70326 (pfn 56d9) for type 20000000
00000000: caf=8000000000000003 taf=2000000000000001
(XEN) mm.c:1439:d0 Failure in alloc_l3_table: entry 0
(XEN) mm.c:2116:d0 Error while validating mfn 74f0f (pfn a2f0) for type 30000000
00000000: caf=8000000000000003 taf=3000000000000001
(XEN) mm.c:2731:d0 Error while pinning mfn 74f0f

For awhile, since I was doing tmem testing, I thought this was a
tmem problem, but I just reproduced it (the first mm.c error message anyway)
with tmem disabled, though the PV guest IS doing self-ballooning.

I'm not sure if this behavior was the same in previous Xen releases
or if this is new behavior.  I've only seen any of this after running
a test for many hours... I tried reproducing it with a shorter similar
test but without luck.

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