linux-kernel - Re: [PATCH v2 0/4] implement vcpu preempted check

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Date:	Thu, 7 Jul 2016 13:09:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>, mpe@...erman.id.au,
	boqun.feng@...il.com, will.deacon@....com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Waiman Long <waiman.long@....com>,
	virtualization@...ts.linux-foundation.org,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>, benh@...nel.crashing.org,
	schwidefsky@...ibm.com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] implement vcpu preempted check

On Thu, Jul 07, 2016 at 11:42:15AM +0200, Peter Zijlstra wrote:
> I suspect you want something like so; except this has holes in.
> 
> We clear KVM_ST_PAD_PREEMPT before disabling preemption and we set it
> after enabling it, this means that if we get preempted in between, the
> vcpu is reported as running even though it very much is not.
> 
> Fixing that requires much larger surgery.

Note that this same hole is already a 'problem' for steal time
accounting. The thread can accrue further delays (iow steal time) after
we've called record_steal_time(). These delays will go unaccounted.