On Friday 26 September 2008 01:42:13 Linus Torvalds wrote: > On Thu, 25 Sep 2008, Rusty Russell wrote: > > This turns out to be awful in practice, mainly due to const. > > Consider: > > > > #ifdef CONFIG_CPUMASK_OFFSTACK > > typedef unsigned long *cpumask_t; > > #else > > typedef unsigned long cpumask_t[1]; > > #endif > > > > cpumask_t returns_cpumask(void); > > No. That's already broken. You cannot return a cpumask_t, regardless of > interface. We must not do it regardless of how we pass those things > around, since it generates _yet_ another temporary on the stack for the > return slot for any kind of structure. No, for large NR_CPUS, cpumask_t is a pointer as shown. And we have numerous basic functions which return a cpumask_t. Yes, this is part of the problem. > What _is_ relevant is how we allocate them when we need temporary CPU > masks. And _that_ is where my suggestion comes in. For small NR_CPUS, we > really do want to allocate them on the stack, because calling kmalloc for > a 4- or 8-byte allocation is just _stupid_. Right, but cpumask_t is used for far more than stack decls, thus the problems. I can make a separate "cpumask_stack_t" and use your method tho. I think that might even reduce churn and allow us to do this in parts. > which has to be converted some way. And I think it needs to be converted > in a way that does *not* force us to call kmalloc() for idiotically small > values. Yeah, got that. But your suggestion to change cpumask_t turned out horribly ugly. Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
