xen/time: do not decrease steal time after live migration on xen
After guest live migration on xen, steal time in /proc/stat
(cpustat[CPUTIME_STEAL]) might decrease because steal returned by
xen_steal_lock() might be less than this_rq()->prev_steal_time which is
derived from previous return value of xen_steal_clock().
For instance, steal time of each vcpu is 335 before live migration.
Since runstate times are cumulative and cleared during xen live migration
by xen hypervisor, the idea of this patch is to accumulate runstate times
to global percpu variables before live migration suspend. Once guest VM is
resumed, xen_get_runstate_snapshot_cpu() would always return the sum of new
runstate times and previously accumulated times stored in global percpu
variables.
Comment above HYPERVISOR_suspend() has been removed as it is inaccurate:
the call can return an error code (e.g., possibly -EPERM in the future).
Similar and more severe issue would impact prior linux 4.8-4.10 as
discussed by Michael Las at
https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest,
which would overflow steal time and lead to 100% st usage in top command
for linux 4.8-4.10. A backport of this patch would fix that issue.
[boris: added linux/slab.h to driver/xen/time.c, slightly reformatted
commit message]
References: https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>