[PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

I observed that sometimes st is 100% instantaneous, then idle is 100% 
even if there is a cpu hog on the guest cpu after the cpu hotplug comes 
back(N.B. both guest and host are latest 4.7-rc1, this can not always 
be readily reproduced). I add trace to capture it as below:

cpuhp/1-12    [001] d.h1   167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0         
cpuhp/1-12    [001] d.h1   167.461659: account_process_tick: steal_jiffies = 1291          
<idle>-0     [001] d.h1   167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000          
<idle>-0     [001] d.h1   167.462664: account_process_tick: steal_jiffies = 18446744072437

The steal clock warps and then steal_jiffies overflow, this patch align 
prev_steal_time to the new steal clock timestamp, in order to avoid 
overflow and st stuff can continue to work.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 kernel/sched/cputime.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 75f98c5..d0eebc3 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -265,7 +265,13 @@ static __always_inline bool steal_account_process_tick(void)
 		unsigned long steal_jiffies;
 
 		steal = paravirt_steal_clock(smp_processor_id());
-		steal -= this_rq()->prev_steal_time;
+		if (likely(steal > this_rq()->prev_steal_time))
+			steal -= this_rq()->prev_steal_time;
+		else {
+			/* steal clock warp */
+			this_rq()->prev_steal_time = steal;
+			return false;
+		}
 
 		/*
 		 * steal is in nsecs but our caller is expecting steal
-- 
1.9.1