[PATCH] sched/cputime: Fix steal time accouting during cpu hotplug

From: Wanpeng Li
Date: Thu Jun 02 2016 - 01:39:21 EST

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")
set rq->prev_* to 0 after a cpu hotplug comes back in order to fix the scenario:

| steal is smaller than rq->prev_steal_time we end up with an insane large
| value which then gets added to rq->prev_steal_time, resulting in a permanent
| wreckage of the accounting.

However, it is still buggy.

rq->prev_steal_time = 0:

As Rik pointed out:

| setting rq->prev_irq_time to 0 in the guest, and then getting a giant value from
| the host, could result in a very large of steal_jiffies.

rq->prev_steal_time_rq = 0:

| steal = paravirt_steal_clock(cpu_of(rq));
| steal -= rq->prev_steal_time_rq;
| if (unlikely(steal > delta))
| steal = delta;
| rq->prev_steal_time_rq += steal;
| delta -= steal;
| rq->clock_task += delta;

steal is a giant value and rq->prev_steal_time_rq is 0, rq->prev_steal_time_rq
grows in delta granularity, rq->clock_task can't ramp up until rq->prev_steal_time_rq
catches up steal clock since delta value will be 0 after reducing steal time from
normal execution time. That's why I obersved that cpuhg/1-12 continue running
until rq->prev_steal_time_rq catches up steal clock timestamp.

I believe rq->prev_irq_time has similar issue. So this patch fix it by setting
rq->prev_* to current irq time and steal clock timestamp after a cpu hotplug
comes back.

Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")'
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
kernel/sched/sched.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 72f1f30..e6758af 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1813,12 +1813,12 @@ static inline void cpufreq_trigger_update(u64 time) {}
static inline void account_reset_rq(struct rq *rq)
- rq->prev_irq_time = 0;
+ rq->prev_irq_time = irq_time_read(cpu_of(rq));
- rq->prev_steal_time = 0;
+ rq->prev_steal_time = paravirt_steal_clock(cpu_of(rq));
- rq->prev_steal_time_rq = 0;
+ rq->prev_steal_time_rq = paravirt_steal_clock(cpu_of(rq));