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

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Message-ID: <56C4B0E1.4090902@hpe.com>
Date:	Wed, 17 Feb 2016 12:41:53 -0500
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 02/17/2016 12:18 PM, Peter Zijlstra wrote:
> 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.

But preempt_disable() is called at the beginning of the spin_lock() 
call. So the additional preempt_disable() in percpu_list_add() is just 
to cover the this_cpu_ptr() call to make sure that the cpu number 
doesn't change. So we are talking about a few ns at most here.

Actually, I think I can remove the preempt_disable() and 
preempt_enable() calls as we just need to put list entry in one of the 
per-cpu lists. It doesn't need to be the same CPU of the current task.

Cheers,
Longman