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

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Date:	Mon, 14 Sep 2015 23:47:23 -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 v6 4/6] locking/pvqspinlock: Collect slowpath lock statistics

On 09/14/2015 05:41 PM, Davidlohr Bueso wrote:
> 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).

Using the per-cpu stats, the overhead should be pretty small as atomic 
instructions are not needed. I would probably need to encapsulate the 
stat code into another header file (e.g. qspinlock_pvstat.h) to avoid 
making the qspinlock_paravirt.h too complex. This will probably be a 
separate patch once this patch series can be merged.

Cheers,
Longman
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