Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

Rusty Russell
2008-06-05T23:59:29Z

On Thursday 05 June 2008 04:18:19 Mike Travis wrote:
> > cpu_local_inc() does all this: it takes the name of a local_t var, and is
> > expected to increment this cpu's version of that. You ripped this out
> > and called it CPU_INC().
>
> Hi,
>
> I'm attempting to test both approaches to compare the object generated in
> order to understand the issues involved here. Here's my code:
>
> void test_cpu_inc(int *s)
> {
> __CPU_INC(s);
> }
>
> void test_local_inc(local_t *t)
> {
> __local_inc(THIS_CPU(t));
> }
>
> void test_cpu_local_inc(local_t *t)
> {
> __cpu_local_inc(t);
> }
>
> But I don't know how I can use cpu_local_inc because the pointer to the
> object is not &__get_cpu_var(l):

Yes. Because the only true per-cpu vars are the static ones, cpu_local_inc()
only works on identifiers, not arbitrary pointers. Once this is fixed, we
should be enhancing the infrastructure to allow that (AFAICT it's not too
hard, but we should add an __percpu marker for sparse).

> At the minimum, we would need a new local_t op to get the correct
> CPU_ALLOC'd pointer value for the increment. These new local_t ops for
> CPU_ALLOC'd variables could use CPU_XXX primitives to implement them, or
> just a base val_to_ptr primitive to replace __get_cpu_var().

I think the latter: __get_cpu_ptr() perhaps?

> I did notice this in local.h:
>
> * X86_64: This could be done better if we moved the per cpu data directly
> * after GS.
>
> ... which it now is, so true per_cpu variables could be optimized better as
> well.

Indeed.

>
> Also, the above cpu_local_wrap(...) adds:
>
> #define cpu_local_wrap(l) \
> ({ \
> preempt_disable(); \
> (l); \
> preempt_enable(); \
> }) \
>
> ... and there isn't a non-preemption version that I can find.

Yes, this should be fixed. I thought i386 had optimized versions pre-merge,
but I was wrong (%gs for per-cpu came later, and noone cleaned up these naive
versions). Did you want me to write them?

I actually think that using local_t == atomic_t is better than
preempt_disable/enable for most archs which can't do atomic deref-and-inc.

> One other distinction is CPU_INC increments an arbitrary sized variable
> while local_inc requires a local_t variable. This may not make it usable
> in all cases.

You might be right, but note that local_t is 64 bit on 64-bit platforms. And
speculation of possible use cases isn't a good reason to rip out working
infrastructure :)

Cheers,
Rusty.

>
> Thanks,
> Mike