Messages in this thread | | | Subject | Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM | From | Eric Dumazet <> | Date | Fri, 10 Dec 2010 21:39:50 +0100 |
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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.
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