Re: [PATCH v2] sched/cputime: add steal clock warp handling

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/06/2016 15:10, Rik van Riel wrote:
> 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>

Sorry for being late---again, I'd like to give a shot to a fix in KVM
guest code.

Paolo