Re: [RFC workqueue/driver-core PATCH 1/5] workqueue: Provide queue_work_near to queue work near a given NUMA node

[Alexander Duyck]

On 10/2/2018 11:41 AM, Tejun Heo wrote:
> Hello,
>
> On Tue, Oct 02, 2018 at 11:23:26AM -0700, Alexander Duyck wrote:
> > > Yeah, it's all in wq_select_unbound_cpu().  Right now, if the
> > > requested cpu isn't in wq_unbound_cpumask, it falls back to dumb
> > > round-robin.  We can probably do better there and find the nearest
> > > node considering topology.
> >
> > Well if we could get wq_select_unbound_cpu doing the right thing
> > based on node topology that would be most of my work solved right
> > there. Basically I could just pass WQ_CPU_UNBOUND with the correct
> > node and it would take care of getting to the right CPU.
>
> Yeah, sth like that.  It might be better to keep the function to take
> cpu for consistency as everything else passes around cpu.
>
> > > > The question I have then is what should I do about workqueues that
> > > > aren't WQ_UNBOUND if they attempt to use queue_work_near? In that
> > >
> > > Hmm... yeah, let's just use queue_work_on() for now.  We can sort it
> > > out later and users could already do that anyway.
> >
> > So are you saying I should just return an error for now if somebody
> > tries to use something other than an unbound workqueue with
> > queue_work_near, and expect everyone else to just use queue_work_on
> > for the other workqueue types?
>
> Oh, I meant that let's not add a new interface for now and just use
> queue_work_on() for your use case too.
>
> Thanks.

So the only issue is that I was hoping to get away with not having to 
add additional preemption. That was the motivation behind doing 
queue_work_near as I could just wrap it all in the same local_irq_save 
that way I don't have to worry about the CPU I am on changing.

What I may look at doing is just greatly reducing the 
workqueue_select_unbound_cpu_near function to essentially just perform a 
few tests and then will just use the results from a cpumask_any_and of 
the cpumask_of_node and the cpu_online_mask. I'll probably rename it 
while I am at it since I am going to probably be getting away from the 
"unbound" checks in the logic.

- Alex