linux-kernel - Re: [PATCH v2 5/6] locking/pvqspinlock: Opportunistically defer kicking to unlock time

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Message-ID: <55A5FA2E.1040406@linux.vnet.ibm.com>
Date:	Wed, 15 Jul 2015 11:44:06 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Waiman Long <Waiman.Long@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v2 5/6] locking/pvqspinlock: Opportunistically defer kicking
 to unlock time

On 07/15/2015 07:43 AM, Waiman Long wrote:
> Performing CPU kicking at lock time can be a bit faster if there
> is no kick-ahead. On the other hand, deferring it to unlock time is
> preferrable when kick-ahead can be performed or when the VM guest is
> having too few vCPUs that a vCPU may be kicked twice before getting
> the lock. This patch implements the deferring kicking when either
> one of the above 2 conditions is true.
>
> Linux kernel builds were run in KVM guest on an 8-socket, 4
> cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
> Haswell-EX system. Both systems are configured to have 32 physical
> CPUs. The kernel build times before and after the patch were:
>
> 		    Westmere			Haswell
>    Patch		32 vCPUs    48 vCPUs	32 vCPUs    48 vCPUs
>    -----		--------    --------    --------    --------
>    Before patch   3m27.4s    10m32.0s	 2m00.8s    14m52.5s
>    After patch	 3m01.3s     9m50.9s	 2m00.5s    13m28.1s
>
> On Westmere, both 32/48 vCPUs case showed some sizeable increase
> in performance. For Haswell, there was some improvement in the
> overcommitted (48 vCPUs) case.
>
> Signed-off-by: Waiman Long <Waiman.Long@...com>
> ---

Hi Waiman,

For virtual guests, from my experiments, lock waiter preemption was the
main concern especially with fair locks.
I find that these set of patches are in right direction to address them.

Thanks for the patches.

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