linux-kernel - Re: [PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

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Message-ID: <20160602120023.GC3190@twins.programming.kicks-ass.net>
Date:	Thu, 2 Jun 2016 14:00:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Radim <rkrcmar@...hat.com>
Subject: Re: [PATCH] sched/cputime: add steal clock warps handling during cpu
 hotplug

On Thu, Jun 02, 2016 at 07:57:19PM +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. 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.

I would rather suggest fixing the steal clock thing to not jump like
that; is that at all possible?