linux-kernel - Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

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Date:	Tue, 7 Jun 2016 13:53:26 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Cc:	Radim Krcmar <rkrcmar@...hat.com>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting
 during cpu hotplug



On 07/06/2016 13:50, Wanpeng Li wrote:
>> > I'm not sure this patch is necessary.  Instead you could just revert
>> > commit e9532e69b8d1.  The previous patch obviously makes it unnecessary
>> > to reset rq->prev_steal_time and rq->prev_steal_time_rq, and the reset
>> > of rq->prev_irq_time looks like a no-op to me.
> The reason why I'm not just simple revert it is that commit mentioned
> "steal is smaller than rq->prev_steal_time we end up with an insane
> large value which then gets added to rq->prev_steal_time, resulting in
> a permanent wreckage of the accounting."

With this patch, you go back to having underflow if steal is smaller
than rq->prev_steal_time.  The point is that it should never be smaller;
it was only smaller because of the bug that you are fixing in patch 1.

Thanks,

Paolo

 Though I didn't meet such
> scenario. So I just do what that commit really want to do.