linux-kernel - Re: [PATCH tip/locking/core v9 4/6] locking/pvqspinlock: Collect slowpath lock statistics

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Message-ID: <20151105164322.GH3604@twins.programming.kicks-ass.net>
Date:	Thu, 5 Nov 2015 17:43:22 +0100
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>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH tip/locking/core v9 4/6] locking/pvqspinlock: Collect
 slowpath lock statistics

On Thu, Nov 05, 2015 at 11:29:29AM -0500, Waiman Long wrote:
> On 11/02/2015 11:40 AM, Peter Zijlstra wrote:
> >On Fri, Oct 30, 2015 at 07:26:35PM -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 data which
> >>enable us to calculate the kicking and wakeup latencies which have
> >>a heavy dependency on the CPUs being used.
> >>
> >>The statistical counters are per-cpu variables to minimize the
> >>performance overhead in their updates. These counters are exported
> >>via the sysfs filesystem under the /sys/kernel/qlockstat directory.
> >>When the corresponding sysfs files are read, summation and computing
> >>of the required data are then performed.
> >Why did you switch to sysfs? You can create custom debugfs files too.
> 
> I was not aware of that capability. So you mean using debugfs_create_file()
> using custom file_operations. Right? 

Yep.

> That doesn't seem to be easier than
> using sysfs. However, I can use that if you think it is better to use
> debugfs.

Mostly I just wanted to point out that it was possible; you need not
change to sysfs because debugfs lacks the capability.

But now that you ask, I think debugfs might be the better place, such
statistics (and the proposed CONFIG symbol) are purely for debug
purposes, right?
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