Date: Wed, 2 Mar 2016 09:41:29 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <andi@firstfloor.org>, Dave Chinner <dchinner@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v5 5/5] lib/percpu-list: Add a config parameter for
 disabling per-cpu list
Message-ID: <20160302084128.GA21779@gmail.com>
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* Waiman Long <Waiman.Long@hpe.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@hpe.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@kernel.org>

Thanks,

	Ingo