On Mon, 2 Jun 2008, Rusty Russell wrote: > > Believe me I have tried to use local_t repeatedly for vm statistics etc. > > It always fails on that issue. > > Frankly, I am finding it increasingly easy to believe that you failed. But > you are blaming the wrong thing. > > There are three implementations of local_t which are obvious. The best is for > architectures which can locate and increment a per-cpu var in one instruction > (eg. x86). Otherwise, using atomic_t/atomic64_t for local_t provides a > general solution. The other general solution would involve > local_irq_disable()/increment/local_irq_enable(). > > My (fading) hope is that this idiocy is an abberation, 1. The x86 implementation does not exist because the segment register has so far not been available on x86_64. So you could not do the solution. You need the zero basing. Then you can use per_xxx_add in cpu_inc. 2. The general solution created overhead that is often not needed. If we would have done vm event counters with local_t then we would have atomic overhead for each increment on f.e. IA64. That was not acceptable. cpu_alloc never falls back to atomic operations. 3. local_t is based on the atomic logic. But percpu handling is fundamentally different in that accesses without the special macros are okay provided you are in a non preemptible or irq context! A local_t declaration makes such accesses impossible. 4. The modeling of local_t on atomic_t limits it to 32bit! There is no way to use this with pointers or 64 bit entities. Adding that would duplicate the API for each type added. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
