linux-kernel - Re: [PATCH v5 5/5] lib/percpu-list: Add a config parameter for disabling per-cpu list

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Date:	Wed, 2 Mar 2016 16:55:04 -0500
From:	Waiman Long <waiman.long@....com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v5 5/5] lib/percpu-list: Add a config parameter for disabling
 per-cpu list

On 03/02/2016 03:41 AM, Ingo Molnar wrote:
> * Waiman Long<Waiman.Long@....com>  wrote:
>
>> As there is concern that the larger pcpu_list_node structure and the
>> per-cpu overhead may be a waste of resource on small system. This patch
>> adds a config parameter CONFIG_PERCPU_LIST to disable the per-cpu list
>> if the kernel builder chooses to do so. With per-cpu list disabled,
>> all the different groups of per-cpu lists will be degenerated into
>> global lists for all the CPUs.
>>
>> The current default is to enable per-cpu list. A kernel builder needs
>> to explicitly turn it off.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@....com>
>> ---
>>   fs/inode.c                  |    2 +-
>>   include/linux/percpu-list.h |   93 ++++++++++++++++++++++++++++++++++++++++++-
>>   lib/Kconfig                 |   14 ++++++
>>   lib/percpu-list.c           |   24 +++++++++++-
>>   4 files changed, 130 insertions(+), 3 deletions(-)
> I think this kind of #ifdef complexity and the doubling of our Kconfig and testing
> space is counterproductive, and I think the per CPU locking is a win on as small
> as dual core CPUs, and on UP CPUs the per CPU list becomes a single global list
> automatically.
>
> I'm not against visible memory savings for overly clever scalability features, but
> this does not appear to be such a case, so:
>
> NAKed-by: Ingo Molnar<mingo@...nel.org>
>
> Thanks,
>
> 	Ingo

The last patch was there to answer a feedback from Jan. I am fine if 
that is not merged as I prefer to have the per-cpu list capability as 
the default anyway.

Cheers,
Longman