Re: [PATCH RFC tip/core/rcu] accelerate grace period if lastnon-dynticked CPU

From: Peter Zijlstra
Date: Wed Jan 27 2010 - 04:51:54 EST

On Wed, 2010-01-27 at 10:43 +0100, Andi Kleen wrote:
> Can't you simply check that at runtime then?
> if (num_possible_cpus() > 20)
> ...
> BTW the new small is large. This years high end desktop PC will come with
> upto 12 CPU threads. It would likely be challenging to find a good
> number for 20 that holds up with the future.

If only scalability were that easy :/

These massive core/thread count things are causing more problems as
well, the cpus/node ratios are constantly growing, giving grief in the
page allocator as well as other places that used to scale per node.

As to the current problem, the call_rcu() interface doesn't make a hard
promise that the callback will be done on the same cpu, right? So why
not simply move the callback list over to a more active cpu?

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