Subject: Re: [PATCH 00/05] robust per_cpu allocation for modules

From: Steven Rostedt <[email-masked]>
Date: Sun, 16 Apr 2006 20:45:50 -0400

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