Messages in this thread |  | | | Date | Tue, 19 Feb 2008 08:02:53 -0500 | | From | Mathieu Desnoyers <> | | Subject | Re: Linux 2.6.25-rc2 |
* Ingo Molnar (mingo@elte.hu) wrote: > > * Pekka Enberg <penberg@cs.helsinki.fi> wrote: > > > Mathieu, Christoph is on vacation and I'm not at all that familiar > > with this cmpxchg_local() optimization, so if you could take a peek at > > this bug report to see if you can spot something obviously wrong with > > it, I would much appreciate that. > > hm, it's bad for at least one other reason as well (which is probably > unrelated to this crash): > > /* > * Currently fastpath is not supported if preemption is enabled. > */ > #if defined(CONFIG_FAST_CMPXCHG_LOCAL) && !defined(CONFIG_PREEMPT) > #define SLUB_FASTPATH > #endif > > such !PREEMPT exceptions tend to show "i didnt want to think too hard > about the preemptible case so just turn it off" thinking. >
Ingo, a comment in slub.c explains it :
/* * The SLUB_FASTPATH path is provisional and is currently disabled if the * kernel is compiled with preemption or if the arch does not support * fast cmpxchg operations. There are a couple of coming changes that will * simplify matters and allow preemption. Ultimately we may end up making * SLUB_FASTPATH the default. * * 1. The introduction of the per cpu allocator will avoid array lookups * through get_cpu_slab(). A special register can be used instead. * * 2. The introduction of per cpu atomic operations (cpu_ops) means that * we can realize the logic here entirely with per cpu atomics. The * per cpu atomic ops will take care of the preemption issues. */ So there is more coming in the preemption area.
> Also, why isnt this "SLUB_FASTPATH" flag done in the Kconfig space? >
Eventually, I think only CONFIG_FAST_CMPXCHG_LOCAL will be needed (when the code will support preemption). Therefore, this SLUB_FASTPATH define seems to be only here temporarily.
I'm looking at the code right now.. more to come.
Mathieu
> Ingo
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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