* 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. 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. 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 -- 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
