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 09:41:29 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <Waiman.Long@....com>
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


* 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