linux-ext4 - Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper functions

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Date:	Thu, 7 Apr 2016 12:06:23 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Waiman Long <waiman.long@....com>
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

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.

-- 
tejun
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