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

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Date:	Mon, 06 Jun 2016 10:22:15 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	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>,
	Radim <rkrcmar@...hat.com>
Subject: Re: [PATCH v2] sched/cputime: add steal clock warp handling

On Mon, 2016-06-06 at 15:44 +0200, Paolo Bonzini wrote:
> 
> 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@...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>
> Sorry for being late---again, I'd like to give a shot to a fix in KVM
> guest code.

This is in the guest code.

Do you mean a fix on the host side?

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