When both stolen and blocked are rounded down, it is possible for the

final increment of the cpu local processed_system_time to move the cpu
local system time ahead a bit further than expected - but still proper
wrt. wall clock time.

This gives problems on a next timer interrupt, when stolen and blocked
both get rounded up and end up incrementing the per cpu
processed_system_time too far.

These simple checks make sure that the cpu local processed_system_time
never gets advanced too far.  Not advancing the variable now should be
fine, since we'll increment it at the next timer tick...

This patch has made the "time went backwards" error messages go away
completely.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Keir Fraser <keir@xensource.com>

diff --git a/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c b/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
index ee5bbe160fc79029e2ca5c5a4b657f42ef9cbbce..62139abbe4cc139139c3e025cf6b375124fa1365 100644 (file)
--- a/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
+++ b/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
@@ -645,7 +645,7 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
             unlikely(delta_cpu < -(s64)permitted_clock_jitter))
            && printk_ratelimit()) {
                printk("Timer ISR/%d: Time went backwards: "
-                      "delta=%lld cpu_delta=%lld shadow=%lld "
+                      "delta=%lld delta_cpu=%lld shadow=%lld "
                       "off=%lld processed=%lld cpu_processed=%lld\n",
                       cpu, delta, delta_cpu, shadow->system_timestamp,
                       (s64)get_nsec_offset(shadow),
@@ -675,8 +675,10 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
         * HACK: Passing NULL to account_steal_time()
         * ensures that the ticks are accounted as stolen.
         */
-       if (stolen > 0) {
+       if ((stolen > 0) && (delta_cpu > 0)) {
                delta_cpu -= stolen;
+               if (unlikely(delta_cpu < 0))
+                       stolen += delta_cpu; /* clamp local-time progress */
                do_div(stolen, NS_PER_TICK);
                per_cpu(processed_stolen_time, cpu) += stolen * NS_PER_TICK;
                per_cpu(processed_system_time, cpu) += stolen * NS_PER_TICK;
@@ -688,8 +690,10 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
         * HACK: Passing idle_task to account_steal_time()
         * ensures that the ticks are accounted as idle/wait.
         */
-       if (blocked > 0) {
+       if ((blocked > 0) && (delta_cpu > 0)) {
                delta_cpu -= blocked;
+               if (unlikely(delta_cpu < 0))
+                       blocked += delta_cpu; /* clamp local-time progress */
                do_div(blocked, NS_PER_TICK);
                per_cpu(processed_blocked_time, cpu) += blocked * NS_PER_TICK;
                per_cpu(processed_system_time, cpu)  += blocked * NS_PER_TICK;