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

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Date:	Fri, 03 Jun 2016 09:10:07 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Radim <rkrcmar@...hat.com>
Subject: Re: [PATCH v2] sched/cputime: add steal clock warp handling

On Fri, 2016-06-03 at 13:21 +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.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@...nel.org>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> 

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

-- 
All Rights Reversed.


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