[PATCH 1/1] sched/cputime: do not decrease steal time after live migration on xen

[Dongli Zhang <dongli.zhang@oracle.com>]

After guest live migration on xen, steal time in /proc/stat
(cpustat[CPUTIME_STEAL]) might decrease because steal returned by
paravirt_steal_clock() might be less than this_rq()->prev_steal_time.

For instance, steal time of each vcpu is 335 before live migration.

cpu  198 0 368 200064 1962 0 0 1340 0 0
cpu0 38 0 81 50063 492 0 0 335 0 0
cpu1 65 0 97 49763 634 0 0 335 0 0
cpu2 38 0 81 50098 462 0 0 335 0 0
cpu3 56 0 107 50138 374 0 0 335 0 0

After live migration, steal time is reduced to 312.

cpu  200 0 370 200330 1971 0 0 1248 0 0
cpu0 38 0 82 50123 500 0 0 312 0 0
cpu1 65 0 97 49832 634 0 0 312 0 0
cpu2 39 0 82 50167 462 0 0 312 0 0
cpu3 56 0 107 50207 374 0 0 312 0 0

The code in this patch is borrowed from do_stolen_accounting() which has
already been removed from linux source code since commit ecb23dc6 ("xen:
add steal_clock support on x86").

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.
Unlike the issue discussed by Michael Las which would overflow steal time
and lead to 100% st usage in top command for linux 4.8-4.10, the issue for
linux 4.11+ would only decrease but not overflow steal time after live
migration.

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>
---
 kernel/sched/cputime.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 14d2dbf..57d09cab 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -238,10 +238,17 @@ static __always_inline u64 steal_account_process_time(u64 maxtime)
 {
 #ifdef CONFIG_PARAVIRT
 	if (static_key_false(&paravirt_steal_enabled)) {
-		u64 steal;
+		u64 steal, steal_time;
+		s64 steal_delta;
+
+		steal_time = paravirt_steal_clock(smp_processor_id());
+		steal = steal_delta = steal_time - this_rq()->prev_steal_time;
+
+		if (unlikely(steal_delta < 0)) {
+			this_rq()->prev_steal_time = steal_time;
+			return 0;
+		}
 
-		steal = paravirt_steal_clock(smp_processor_id());
-		steal -= this_rq()->prev_steal_time;
 		steal = min(steal, maxtime);
 		account_steal_time(steal);
 		this_rq()->prev_steal_time += steal;
-- 
2.7.4