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

[Waiman Long]

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