Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-27T20:22:20Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Mon, 27 Aug 2007, Peter Zijlstra wrote:
>
> > So, if the fast path can be done with a preempt off, it might be doable
> > to suffer the slow path with a per cpu lock like that.
>
> Sadly the cmpxchg_local requires local per cpu data access. Isnt there
> some way to make this less expensive on RT? Acessing cpu local memory is
> really good for performance on NUMA since the data is optimally placed and
> one can avoid/reduce locking if the process stays tied to the processor.
>

On the slow path, in slab_new, we already have to reenable interrupts
because we can sleep. If we make sure that whenever we return to an irq
disable code path we take the current per-cpu data structure again, can
we make the preempt-disable/irq-disabled code paths O(1) ?