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
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