Re: [PATCH] SLUB: use have_arch_cmpxchg()

Christoph Lameter
2007-08-27T19:43:34Z

On Mon, 27 Aug 2007, Mathieu Desnoyers wrote:

> Hrm, actually, I don't think such have_arch_cmpxchg() macro will be
> required at all because of the small performance hit disabling
> preemption will have on the slow and fast paths. Let's compare, for each
> of the slow path and fast path, what locking looks like on architecture
> with and without local cmpxchg:
>
> What we actually do here is:
>
> fast path:
> with local_cmpxchg:
> preempt_disable()
> preempt_enable()
> without local_cmpxchg:
> preempt_disable()
> local_irq_save
> local_irq_restore
> preempt_enable()
> (we therefore disable preemption _and_ disable interrupts for
> nothing)

Hmmm..... This is a performance issue for preempt kernels.

> slow path:
> both with and without local_cmpxchg():
> preempt_disable()
> preempt_enable()

Here we potentially loose our per cpu structure since the process may be
rescheduled.

> local_irq_save()
> local_irq_restore()
>
>
> Therefore, we would add preempt disable/enable to the fast path of
> architectures where local_cmpxchg is emulated with irqs off. But since
> preempt disable/enable is just a check counter increment/decrement with
> barrier() and thread flag check, I doubt it would hurt performances
> enough to justify the added complexity of disabling interrupts for the
> whole fast path in this case.

One additional cacheline referenced in the hot path. Ok the counter may be
hot as well if this is a preemptible kernel. Nevertheless a cause for
concern.