linux-kernel - Re: [PATCH] kvm: handle last_boosted

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Message-ID: <20120706171128.GA8835@amt.cnet>
Date:	Fri, 6 Jul 2012 14:11:28 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Avi Kivity <avi@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: handle last_boosted_vcpu = 0 case

On Tue, Jun 19, 2012 at 04:51:04PM -0400, Rik van Riel wrote:
> On Wed, 20 Jun 2012 01:50:50 +0530
> Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> wrote:
> 
> > 
> > In ple handler code, last_boosted_vcpu (lbv) variable is
> > serving as reference point to start when we enter.
> 
> > Also statistical analysis (below) is showing lbv is not very well
> > distributed with current approach.
> 
> You are the second person to spot this bug today (yes, today).
> 
> Due to time zones, the first person has not had a chance yet to
> test the patch below, which might fix the issue...
> 
> Please let me know how it goes.
> 
> ====8<====
> 
> If last_boosted_vcpu == 0, then we fall through all test cases and
> may end up with all VCPUs pouncing on vcpu 0.  With a large enough
> guest, this can result in enormous runqueue lock contention, which
> can prevent vcpu0 from running, leading to a livelock.
> 
> Changing < to <= makes sure we properly handle that case.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>

Applied, thanks.

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