[PATCH v3 1/1] xen/time: do not decrease steal time after live migration on xen

[Dongli Zhang]

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.

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

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.

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.

References: https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

---
Changed since v1:
  * relocate modification to xen_get_runstate_snapshot_cpu

Changed since v2:
  * accumulate runstate times before live migration

---
 drivers/xen/manage.c  |  1 +
 drivers/xen/time.c    | 19 +++++++++++++++++++
 include/xen/xen-ops.h |  1 +
 3 files changed, 21 insertions(+)

diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
index c425d03..9aa2955 100644
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -72,6 +72,7 @@ static int xen_suspend(void *data)
 	}
 
 	gnttab_suspend();
+	xen_accumulate_runstate_time();
 	xen_arch_pre_suspend();
 
 	/*
diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index ac5f23f..6df3f82 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -19,6 +19,8 @@
 /* runstate info updated by Xen */
 static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);
 
+static DEFINE_PER_CPU(u64[4], old_runstate_time);
+
 /* return an consistent snapshot of 64-bit time/counter value */
 static u64 get64(const u64 *p)
 {
@@ -52,6 +54,7 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
 {
 	u64 state_time;
 	struct vcpu_runstate_info *state;
+	int i;
 
 	BUG_ON(preemptible());
 
@@ -64,6 +67,22 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
 		rmb();	/* Hypervisor might update data. */
 	} while (get64(&state->state_entry_time) != state_time ||
 		 (state_time & XEN_RUNSTATE_UPDATE));
+
+	for (i = 0; i < 4; i++)
+		res->time[i] += per_cpu(old_runstate_time, cpu)[i];
+}
+
+void xen_accumulate_runstate_time(void)
+{
+	struct vcpu_runstate_info state;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		xen_get_runstate_snapshot_cpu(&state, cpu);
+		memcpy(per_cpu(old_runstate_time, cpu),
+				state.time,
+				4 * sizeof(u64));
+	}
 }
 
 /*
diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
index 218e6aa..5680059 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -32,6 +32,7 @@ void xen_resume_notifier_unregister(struct notifier_block *nb);
 bool xen_vcpu_stolen(int vcpu);
 void xen_setup_runstate_info(int cpu);
 void xen_time_setup_guest(void);
+void xen_accumulate_runstate_time(void);
 void xen_get_runstate_snapshot(struct vcpu_runstate_info *res);
 u64 xen_steal_clock(int cpu);
 
-- 
2.7.4