From: Waiman Long <waiman.long@hpe.com>
To: Tejun Heo <tj@kernel.org>
Cc: "Theodore Ts'o" <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Christoph Lameter <cl@linux.com>, <linux-ext4@vger.kernel.org>,
<linux-kernel@vger.kernel.org>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>,
Toshimitsu Kani <toshi.kani@hpe.com>
Subject: Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper functions
Date: Thu, 7 Apr 2016 14:52:33 -0400 [thread overview]
Message-ID: <5706AC71.3080801@hpe.com> (raw)
In-Reply-To: <20160407160623.GF7822@mtj.duckdns.org>
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
next prev parent reply other threads:[~2016-04-07 19:07 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-02 3:09 [PATCH 0/3] ext4: Improve parallel I/O performance on NVDIMM Waiman Long
2016-04-02 3:09 ` [PATCH 1/3] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio_begin() called Waiman Long
2016-04-02 3:09 ` [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper functions Waiman Long
2016-04-04 7:36 ` Nikolay Borisov
2016-04-04 17:11 ` Waiman Long
2016-04-04 19:09 ` Christoph Lameter
2016-04-06 21:53 ` Waiman Long
2016-04-04 16:02 ` Tejun Heo
2016-04-06 19:52 ` Waiman Long
2016-04-06 21:51 ` Waiman Long
2016-04-06 22:54 ` Tejun Heo
2016-04-07 15:58 ` Waiman Long
2016-04-07 16:06 ` Tejun Heo
2016-04-07 18:52 ` Waiman Long [this message]
2016-04-07 18:58 ` Tejun Heo
2016-04-07 20:37 ` Waiman Long
2016-04-07 20:41 ` Tejun Heo
2016-04-07 21:38 ` Waiman Long
2016-04-02 3:09 ` [PATCH 3/3] ext4: Make cache hits/misses per-cpu counts Waiman Long
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