Re: [PATCH] stack and rcu interaction bug in smp_call_function_mask()

[Jeremy Fitzhardinge]

Nick Piggin wrote:
> Well that's implemented with the optimized call-single code of course,
> so it could be used to implement the masked calls...
>
> I had wanted to look into finding a good cutoff point and use the
> percpu queues for light weight masks, and the single global queue for
> larger ones.
>
> Queue per cpu is not going to be perfect, though. In the current
> implementation, you would need a lot of data structures. You could
> alleviate this problem by using per CPU vectors rather than lists,
> but then you get the added problem of resource starvation at the
> remote end too.
>
> For heavy weight masks on large systems, the single queue I'd say
> will be a win. But I never did detailed measurements, so I'm open
> to be proven wrong.
>   

Yeah, there's a lot of parameters there.  And as I've mentioned before, 
I wonder whether we should take NUMA topology into account when deciding 
where and when to use queues.  My intuition is that most cross-cpu calls 
are going to be within cpus on a node, on the grounds that most are 
mm->cpu_vm_mask calls, and the rest of the system tries hard to 
co-locate processes sharing memory on one node.

Waffle, handwave.

   J
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