linux-kernel - Re: [PATCH v3 3/7] locking/pvqspinlock: Collect slowpath lock statistics

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Date:	Mon, 27 Jul 2015 13:33:53 -0400
From:	Waiman Long <waiman.long@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
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>
Subject: Re: [PATCH v3 3/7] locking/pvqspinlock: Collect slowpath lock statistics

On 07/26/2015 09:14 PM, Davidlohr Bueso wrote:
> On Wed, 2015-07-22 at 16:12 -0400, Waiman Long wrote:
>> This patch enables the accumulation of kicking and waiting related
>> PV qspinlock statistics when the new QUEUED_LOCK_STAT configuration
>> option is selected. It also enables the collection of kicking and
>> wakeup latencies which have a heavy dependency on the CPUs being used.
>>
>> The measured latencies for different CPUs are:
>>
>> 	CPU		Wakeup		Kicking
>> 	---		------		-------
>> 	Haswell-EX	89.8us		 7.4us
>> 	Westmere-EX	67.6us		 9.3us
>>
> But you don't mention anything about the overhead of enabling
> QUEUED_LOCK_STAT. This does several atomic ops, thus potentially
> thrashing workloads.
>

Yes, QUEUED_LOCK_STAT will slow performance a bit. It is like enabling 
LOCK_STAT and you will expect some slow down too. It is essentially a 
debugging option to see what had actually happened in the system. It 
should be turned off in a production system.

Cheers,
Longman
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