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Subject[PATCH 08/12] xen/time: introduce xen_{save,restore}_steal_clock
From: Munehisa Kamata <kamatam@amazon.com>

Currently, steal time accounting code in scheduler expects steal clock
callback to provide monotonically increasing value. If the accounting
code receives a smaller value than previous one, it uses a negative
value to calculate steal time and results in incorrectly updated idle
and steal time accounting. This breaks userspace tools which read
/proc/stat.

top - 08:05:35 up 2:12, 3 users, load average: 0.00, 0.07, 0.23
Tasks: 80 total, 1 running, 79 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 0.0%sy, 0.0%ni,30100.0%id, 0.0%wa, 0.0%hi, 0.0%si,-1253874204672.0%st

This can actually happen when a Xen PVHVM guest gets restored from
hibernation, because such a restored guest is just a fresh domain from
Xen perspective and the time information in runstate info starts over
from scratch.

This patch introduces xen_save_steal_clock() which saves current values
in runstate info into per-cpu variables. Its couterpart,
xen_restore_steal_clock(), sets offset if it found the current values in
runstate info are smaller than previous ones. xen_steal_clock() is also
modified to use the offset to ensure that scheduler only sees
monotonically increasing number.

Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
Signed-off-by: Anchal Agarwal <anchalag@amazon.com>
---
drivers/xen/time.c | 29 ++++++++++++++++++++++++++++-
include/xen/xen-ops.h | 2 ++
2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index 0968859c29d0..3560222cc0dd 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -23,6 +23,9 @@ static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);

static DEFINE_PER_CPU(u64[4], old_runstate_time);

+static DEFINE_PER_CPU(u64, xen_prev_steal_clock);
+static DEFINE_PER_CPU(u64, xen_steal_clock_offset);
+
/* return an consistent snapshot of 64-bit time/counter value */
static u64 get64(const u64 *p)
{
@@ -149,7 +152,7 @@ bool xen_vcpu_stolen(int vcpu)
return per_cpu(xen_runstate, vcpu).state == RUNSTATE_runnable;
}

-u64 xen_steal_clock(int cpu)
+static u64 __xen_steal_clock(int cpu)
{
struct vcpu_runstate_info state;

@@ -157,6 +160,30 @@ u64 xen_steal_clock(int cpu)
return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline];
}

+u64 xen_steal_clock(int cpu)
+{
+ return __xen_steal_clock(cpu) + per_cpu(xen_steal_clock_offset, cpu);
+}
+
+void xen_save_steal_clock(int cpu)
+{
+ per_cpu(xen_prev_steal_clock, cpu) = xen_steal_clock(cpu);
+}
+
+void xen_restore_steal_clock(int cpu)
+{
+ u64 steal_clock = __xen_steal_clock(cpu);
+
+ if (per_cpu(xen_prev_steal_clock, cpu) > steal_clock) {
+ /* Need to update the offset */
+ per_cpu(xen_steal_clock_offset, cpu) =
+ per_cpu(xen_prev_steal_clock, cpu) - steal_clock;
+ } else {
+ /* Avoid unnecessary steal clock warp */
+ per_cpu(xen_steal_clock_offset, cpu) = 0;
+ }
+}
+
void xen_setup_runstate_info(int cpu)
{
struct vcpu_register_runstate_memory_area area;
diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
index 89b1e88712d6..74fb5eb3aad8 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -37,6 +37,8 @@ void xen_time_setup_guest(void);
void xen_manage_runstate_time(int action);
void xen_get_runstate_snapshot(struct vcpu_runstate_info *res);
u64 xen_steal_clock(int cpu);
+void xen_save_steal_clock(int cpu);
+void xen_restore_steal_clock(int cpu);

int xen_setup_shutdown_event(void);

--
2.24.1.AMZN
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