Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

Eric Dumazet
2010-12-10T20:39:56Z

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.