linux-kernel - Re: [PATCH 3/7] locking/pvqspinlock: Implement wait-early for overcommitted guest

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Message-ID: <20150712082307.GT19282@twins.programming.kicks-ass.net>
Date:	Sun, 12 Jul 2015 10:23:07 +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 3/7] locking/pvqspinlock: Implement wait-early for
 overcommitted guest

On Sat, Jul 11, 2015 at 04:36:54PM -0400, Waiman Long wrote:
> In an overcommitted guest where some vCPUs have to be halted to make
> forward progress in other areas, it is highly likely that a vCPU
> later in the spinlock queue will be spinning while the ones earlier in
> the queue would have been halted already. The spinning in the later
> vCPUs is then just a waste of precious CPU cycles because they are
> not going to get the lock soon as the earlier ones have to be woken
> up and take their turn to get the lock.
> 
> This patch implements a wait-early mechanism where the vCPU will
> call pv_wait() earlier if the previous vCPU is in the halted state
> already. In this case, it will spin less before calling pv_wait(). On
> the other hand, if the previous vCPU was running and then becomes
> halted, the current vCPU will call pv_wait() immmediately in this case.
> 
> This patch also separates the spin threshold for queue head and
> queue nodes.  It favors the queue head by allowing it to spin longer
> before calling pv_wait().

-ENONUMBERS
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