Re: [PATCH] rcu: Use cpus_read_lock() while looking at cpu_online_mask

[Paul E. McKenney]

On Mon, Oct 15, 2018 at 11:33:48PM +0800, Boqun Feng wrote:
> On Mon, Oct 15, 2018 at 05:09:03PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2018-10-15 23:07:15 [+0800], Boqun Feng wrote:
> > > Hi, Sebastian
> > Hi Boqun,
> > 
> > > On Mon, Oct 15, 2018 at 04:42:17PM +0200, Sebastian Andrzej Siewior wrote:
> > > > On 2018-10-13 06:48:13 [-0700], Paul E. McKenney wrote:
> > > > > 
> > > > > My concern would be that it would queue it by default for the current
> > > > > CPU, which would serialize the processing, losing the concurrency of
> > > > > grace-period initialization.  But that was a long time ago, and perhaps
> > > > > workqueues have changed. 
> > > > 
> > > > but the code here is always using the first CPU of a NUMA node or did I
> > > > miss something?
> > > > 
> > > 
> > > The thing is the original way is to pick one CPU for a *RCU* node to
> > > run the grace-period work, but with your proposal, if a RCU node is
> > > smaller than a NUMA node (having fewer CPUs), we could end up having two
> > > grace-period works running on one CPU. I think that's Paul's concern.
> > 
> > Ah. Okay. From what I observed, the RCU nodes and NUMA nodes were 1:1
> > here. Noted.
> 
> Ok, in that case, there should be no significant performance difference.
> 
> > Given that I can enqueue a work item on an offlined CPU I don't see why
> > commit fcc6354365015 ("rcu: Make expedited GPs handle CPU 0 being
> > offline") should make a difference. Any objections to just revert it?
> 
> Well, that commit is trying to avoid queue a work on an offlined CPU,
> because according to workqueue API, it's the users' responsibility to
> make sure the CPU is online when a work item enqueued. So there is a
> difference ;-)
> 
> But I don't have any objection to revert it with your proposal, since
> yours is more simple and straight-forward, and doesn't perform worse if
> NUMA nodes and RCU nodes have one-to-one corresponding.
> 
> Besides, I think even if we observe some performance difference in the
> future, the best way to solve that is to make workqueue have a more
> fine-grained affine group than a NUMA node.

Please keep in mind that there are computer systems out there with NUMA
topologies that are completely incompatible with RCU's rcu_node tree
structure.  According to Rik van Riel (CCed), there are even systems
out there where CPU 0 is on socket 0, CPU 1 on socket 1, and so on,
round-robining across the sockets.

The system that convinced me that the additional constraints on
the workqueue's CPU had CPUs 0-7 on one socket and CPUs 8-15 on the
second, and with CPUs 0-15 sharing the same leaf rcu_node structure.
Unfortunately, I no longer have useful access to this system (dead disk
drive, apparently).

I am not saying that Sebastian's approach is bad, rather that it does
need to be tested on a variety of systems.

							Thanx, Paul

> Regards,
> Boqun
> 
> > 
> > > Regards,
> > > Boqun
> > 
> > Sebastian