linux-kernel - Re: [PATCH 2/7] locking/pvqspinlock: Allow vCPUs kick-ahead

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Message-ID: <20150713135220.GI3644@twins.programming.kicks-ass.net>
Date:	Mon, 13 Jul 2015 15:52:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 2/7] locking/pvqspinlock: Allow vCPUs kick-ahead

On Sat, Jul 11, 2015 at 04:36:53PM -0400, Waiman Long wrote:
> Frequent CPU halting (vmexit) and CPU kicking (vmenter) lengthens
> critical section and block forward progress.  This patch implements
> a kick-ahead mechanism where the unlocker will kick the queue head
> vCPUs as well as up to two additional vCPUs next to the queue head if
> they were halted.  The kickings are done after exiting the critical
> section to improve parallelism.
> 
> The amount of kick-ahead allowed depends on the number of vCPUs in
> the VM guest.  This change should improve overall system performance
> in a busy overcommitted guest.

-ENONUMBERS... also highly workload sensitive, if the lock hold time is
just above our spin time you're wasting gobs of runtime.
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