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