On Fri, 2016-06-03 at 13:21 +0800, Wanpeng Li wrote:
> 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. 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 warp and then steal_jiffies overflow.
> 
> Rik also pointed out to me:
>  
> > 
> > I have seen stuff like that with live migration too, in the past 
> This patch adds steal clock warp handling by a safe threshold to
> only 
> apply steal times that are positive and smaller than one second (as 
> long as nohz_full has the one second timer tick left), ignoring
> intervals 
> that are negative or longer than a second, and using those to sync
> up 
> the guest with the host.
> 
> 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>
> 

Reviewed-by: Rik van Riel <riel@redhat.com>

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