[BUG] 2.6.37-rc3 massive interactivity regression on ARM

[Eric Dumazet]

Le vendredi 10 décembre 2010 à 14:23 -0600, Christoph Lameter a écrit :
> On Fri, 10 Dec 2010, Peter Zijlstra wrote:
> 
> > Its not about passing per-cpu pointers, its about passing long pointers.
> >
> > When I write:
> >
> > void foo(u64 *bla)
> > {
> > 	*bla++;
> > }
> >
> > DEFINE_PER_CPU(u64, plop);
> >
> > void bar(void)
> > {
> > 	foo(__this_cpu_ptr(plop));
> > }
> >
> > I want gcc to emit the equivalent to:
> >
> > __this_cpu_inc(plop); /* incq %fs:(%0) */
> >
> > Now I guess the C type system will get in the way of this ever working,
> > since a long pointer would have a distinct type from a regular
> > pointer :/
> >
> > The idea is to use 'regular' functions with the per-cpu data in a
> > transparent manner so as not to have to replicate all logic.
> 
> That would mean you would have to pass information in the pointer at
> runtime indicating that this particular pointer is a per cpu pointer.
> 
> Code for the Itanium arch can do that because it has per cpu virtual
> mappings. So you define a virtual area for per cpu data and then map it
> differently for each processor. If we would have a different page table
> for each processor then we could avoid using segment register and do the
> same on x86.
> 
> > > Seems that you do not have that use case in mind. So a seqlock restricted
> > > to a single processor? If so then you wont need any of those smp write
> > > barriers mentioned earlier. A simple compiler barrier() is sufficient.
> >
> > The seqcount is sometimes read by different CPUs, but I don't see why we
> > couldn't do what Eric suggested.
> 
> But you would have to define a per cpu seqlock. Each cpu would have
> its own seqlock. Then you could have this_cpu_read_seqcount_begin and
> friends:
> 
> 

Yes. It was the idea.

> DEFINE_PER_CPU(seqcount, bla);
> 
> 

This is in Peter patch :)

> 
> 
> /* Start of read using pointer to a sequence counter only.  */
> static inline unsigned this_cpu_read_seqcount_begin(const seqcount_t __percpu *s)
> {
>         /* No other processor can be using this lock since it is per cpu*/
>         ret = this_cpu_read(s->sequence);
> 	barrier();
>         return ret;
> }
> 
> /*
>  * Test if reader processed invalid data because sequence number has changed.
>  */
> static inline int this_cpu_read_seqcount_retry(const seqcount_t __percpu *s, unsigned start)
> {
> 	barrier();
>         return this_cpu_read(s->sequence) != start;
> }
> 
> 
> /*
>  * Sequence counter only version assumes that callers are using their
>  * own mutexing.
>  */
> static inline void this_cpu_write_seqcount_begin(seqcount_t __percpu *s)
> {
>         __this_cpu_inc(s->sequence);
> 	barrier();
> }
> 
> static inline void this_cpuwrite_seqcount_end(seqcount_t __percpu *s)
> {
>         __this_cpu_dec(s->sequence);
> 	barrier();
> }
> 
> 
> Then you can do
> 
> this_cpu_read_seqcount_begin(&bla)
> 
> ...

This was exactly my suggestion Christoph.

I am glad you understand it now.