linux-kernel - Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Message-ID: <20160217171845.GK6357@twins.programming.kicks-ass.net>
Date:	Wed, 17 Feb 2016 18:18:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Dave Chinner <david@...morbit.com>,
	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>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

On Wed, Feb 17, 2016 at 12:12:57PM -0500, Waiman Long wrote:
> On 02/17/2016 11:27 AM, Christoph Lameter wrote:
> >On Wed, 17 Feb 2016, Waiman Long wrote:
> >
> >>I know we can use RCU for singly linked list, but I don't think we can use
> >>that for doubly linked list as there is no easy way to make atomic changes to
> >>both prev and next pointers simultaneously unless you are taking about 16b
> >>cmpxchg which is only supported in some architecture.
> >But its supported in the most important architecutes. You can fall back to
> >spinlocks on the ones that do not support it.
> >
> 
> I guess with some limitations on how the lists can be traversed, we may be
> able to do that with RCU without lock. However, that will make the code more
> complex and harder to verify. Given that in both my and Dave's testing that
> contentions with list insertion and deletion are almost gone from the perf
> profile when they used to be a bottleneck, is it really worth the effort to
> do such a conversion?

My initial concern was the preempt disable delay introduced by holding
the spinlock over the entire iteration.

There is no saying how many elements are on that list and there is no
lock break.