linux-kernel - Re: [PATCH v6 4/6] locking/pvqspinlock: Collect slowpath lock statistics

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Date:	Mon, 14 Sep 2015 14:41:05 -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 v6 4/6] locking/pvqspinlock: Collect slowpath lock
 statistics

On Mon, 14 Sep 2015, Waiman Long wrote:

>You can't use debugfs if we want to have per-cpu stats. We will have 
>to use sysfs instead. This will require more code changes. It is 
>certainly doable, but we have to choose between simplicity and 
>performance overhead. Right now, I am assuming that lock PV lockstat 
>is used primarily for debugging purpose and won't be enabled on 
>production system. If we want to have this capability in production 
>systems, we will certainly need to change it to per-cpu stats and use 
>sysfs instead.
>
>The original PV ticketlock code used debugfs and I was just following 
>its footstep. Do you think it is worthwhile to have this capability 
>available on production system by default?

If we can prove that the overhead is small enough, and do it correctly
(ie see how we do vmstats), it would be _very_ useful data to have
enabled by default for debugging performance issues; methinks. But right
now we have nowhere near that kind of data, not even with this atomic
variant -- although I recall you did mention a workload in a previous
iteration (which would be good to have in the changelog).
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