Re: [PATCH] SLUB use cmpxchg_local

Christoph Lameter
2007-08-28T19:42:59Z

On Tue, 28 Aug 2007, Mathieu Desnoyers wrote:

> Ok, I just had a look at ia64 instruction set, and I fear that cmpxchg
> must always come with the acquire or release semantic. Is there any
> cmpxchg equivalent on ia64 that would be acquire and release semantic
> free ? This implicit memory ordering in the instruction seems to be
> responsible for the slowdown.

No. There is no cmpxchg used in the patches that I tested. The slowdown
seem to come from the need to serialize at barriers. Adding an interrupt
enable/disable in the middle of the hot path creates another serialization
point.

> If such primitive does not exist, then we should think about an irq
> disable fallback for this local atomic operation. However, I would
> prefer to let the cmpxchg_local primitive be bound to the "slow"
> cmpxchg_acq and create something like _cmpxchg_local that would be
> interrupt-safe, but not reentrant wrt NMIs.

Ummm... That is what I did. See the included patch that you quoted. The
measurements show that such a fallback is not preserving the performance
on IA64.