Messages in this thread | | | Date | Mon, 15 Oct 2018 23:33:48 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH] rcu: Use cpus_read_lock() while looking at cpu_online_mask |
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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.
Regards, Boqun
> > > Regards, > > Boqun > > Sebastian [unhandled content-type:application/pgp-signature] | |