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

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Message-ID: <1437959674.25997.30.camel@stgolabs.net>
Date:	Sun, 26 Jul 2015 18:14:34 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
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>
Subject: Re: [PATCH v3 3/7] locking/pvqspinlock: Collect slowpath lock
 statistics

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.

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