Ingo Molnar wrote: > * David Miller <davem@xxxxxxxxxxxxx> wrote: > >> From: Ingo Molnar <mingo@xxxxxxx> >> Date: Tue, 26 Aug 2008 09:22:20 +0200 >> >>> And i guess the next generation of 4K CPUs support should just get away >>> from cpumask_t-on-kernel-stack model altogether, as the current model is >>> not maintainable. We tried the on-kernel-stack variant, and it really >>> does not work reliably. We can fix this in v2.6.28. >> I recenetly did some work on sparc64 to use cpumask pointers as much >> as possible. >> >> The only case that didn't work was due to a limitation in arch >> interfaces for the new generic smp_call_function() code. It passes a >> cpumask_t instead of a pointer to one via >> arch_send_call_function_ipi(). >> >> But other than that, the whole sparc64 SMP stuff uses cpumask_t >> pointers only. > > nice! > >> What it comes down to is that you have to do the "self cpu" and other >> tests in the cross-call dispatch routines themselves, instead of at >> the top-level working on cpumask_t objects. >> >> Otherwise you have to modify cpumask_t objects and thus pluck them >> onto the stack where they take up silly amounts of space. > > What we did was this: we added MAXSMP which just revs up all the SMP > tunables to the maximum, so that we can see any problems early in > testing. > > And we triggered problems, and we fixed a couple of regressions all > around stack footprint. But we didnt catch all of them - some were gcc > version dependent and configuration dependent. So i think it's safe to > say that the whole concept of allowing such a large cpumask_t to be on > the stack is fragile. Iirc, it was the problem of basing percpu variables at zero that hit problems with various gcc toolset versions. I don't remember any version problems with cpumask's on the stack, they all failed the same way... :-) > > Hence, i think the best way forward is to change the whole cpumask_t > concept and disallow explicit masks altogether. It's so easy to smack a > cpumask_t variable on the stack and nothing really warns about it, and > any function can become part of a nested call sequence. This is a great idea! > > So i think the dynamics of it has to be changed: we need a get/put API > and we need to make on-stack cpumask illegal on the build level (in > generic code at least). This has been Rusty's main argument early on i > think, and i now concur. > > Ingo Removing cpumask_t's from the stack is fairly straight forward. The problem of changing all functions to expect a cpumask pointer via a global change is much more problematic. And of course all those functions that return a cpumask value would need to be addressed. Thanks, Mike -- 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
