linux-kernel - Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper functions

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Date:	Thu, 7 Apr 2016 14:52:33 -0400
From:	Waiman Long <waiman.long@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Christoph Lameter <cl@...ux.com>, <linux-ext4@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Toshimitsu Kani <toshi.kani@....com>
Subject: Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper
 functions

On 04/07/2016 12:06 PM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Thu, Apr 07, 2016 at 11:58:13AM -0400, Waiman Long wrote:
>> We can certainly make it watertight. However, that will certainly require
>> adding performance overhead in the percpu stats update fast path which I am
>> not willing to pay.
> There are multiple options depending on the specific balance you want
> to hit.  Reset can be made very heavy (involving RCU sync operation)
> to make hot path overhead minimal, local locking or atomic ops can
> also be used which while more expensive than this_cpu_*() ops still
> avoids cacheline bouncing.
>
>> The purpose of this stat counters reset functionality is to allow developers
>> to reset the stat counters, run certain workload and see how things are
>> going in the kernel when the workload completes assuming that those stat
>> counters are exposed via sysfs, debugfs, etc. The developers can certainly
>> check the stat counters after the reset to make sure that they are properly
>> reset. So I don't think we need an airtight way of doing it. If you have
>> scenarios in your mind that require airtight reset of the stat counters,
>> please let me know and I will see what I can do about it.
> No matter what, don't create something which can yield a completely
> surprising result once in a blue moon.  You might think it's okay
> because the likelihood is low but that just means that the resulting
> malfunctions will be that much more obscure and difficult to
> reproduce.
>
> Thanks.
>

As long as atomic reset is an optional feature that caller can choose at 
init time, I am OK to provide this functionality. I just don't want it 
to be the default because of the performance overhead.

Cheers,
Longman