On Sun, 2006-04-16 at 17:34 +0200, Arnd Bergmann wrote: > On Sunday 16 April 2006 15:40, Steven Rostedt wrote: > > I'll think more about this, but maybe someone else has some crazy ideas > > that can find a solution to this that is both fast and robust. > > Ok, you asked for a crazy idea, you're going to get it ;-) > > You could take a fixed range from the vmalloc area (e.g. 1MB per cpu) > and use that to remap pages on demand when you need per cpu data. > > #define PER_CPU_BASE 0xe000000000000000UL /* arch dependant */ > #define PER_CPU_SHIFT 0x100000UL > #define __per_cpu_offset(__cpu) (PER_CPU_BASE + PER_CPU_STRIDE * (__cpu)) > #define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset(cpu))) > #define __get_cpu_var(var) per_cpu(var, smp_processor_id()) > > This is a lot like the current sparc64 implementation already is. > Hmm, interesting idea. > The tricky part here is the remapping of pages. You'd need to > alloc_pages_node() new pages whenever the already reserved space is > not enough for the module you want to load and then map_vm_area() > them into the space reserved for them. > > Advantages of this solution are: > - no dependant load access for per_cpu() > - might be flexible enough to implement a faster per_cpu_ptr() > - can be combined with ia64-style per-cpu remapping > > Disadvantages are: > - you can't use huge tlbs for mapping per cpu data like the > regular linear mapping -> may be slower on some archs > - does not work in real mode, so percpu data can't be used > inside exception handlers on some architectures. This is probably a big issue. I believe interrupt context in hrtimers uses per_cpu variables. > - memory consumption is rather high when PAGE_SIZE is large That's also something that I'm trying to solve. To use the least amount of memory and still have the performance. Now, I've also thought about allocating per_cpu and when a module is loaded, reallocate more memory and copy it again. Use something like the kstopmachine to sync the system so that the CPUS don't update any per_cpu variables while this is happening, so that things can't get out of sync. This shouldn't be too much of an issue, since this would only be done when a module is being loaded, and that is a user event that doesn't happen often. We would still need to use the method of keeping track of what is allocated and freed, so that when a module is unloaded, we can still free the area in the per_cpu data. And reallocate that area if a module is added that uses less or the same amount of memory as what was freed. -- Steve
