Messages in this thread | | | Date | Thu, 15 Jan 2009 13:26:11 +0100 | From | Ingo Molnar <> | Subject | Re: [patch] add optimized generic percpu accessors |
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* Tejun Heo <tj@kernel.org> wrote:
> Hello, > > Ingo Molnar wrote: > > The new ops are a pretty nice and clean solution i think. > > > > Firstly, accessing the current CPU is the only safe shortcut anyway (there > > is where we can do %fs/%gs / rip-relative addressing modes), and the > > generic per_cpu() APIs dont really provide that guarantee for us. We might > > be able to hook into __get_cpu_var() but those both require to be an > > lvalue and are also relatively rarely used. > > > > So introducing the new, rather straightforward APIs and using them > > wherever they matter for performance is good. Your patchset already shaved > > off an instruction from ordinary per_cpu() accesses, so it's all moving > > rather close to the most-optimal situation already. > > Yeah, I don't think we can do much better than those ops. I have two > issues tho. > > 1. percpu_and() is missing. I added it for completeness's sake.
Sure - it would be commonly used as well. Perhaps we dont need percpu_xor() at all? (or and and ops already give a complete algebra)
> 2. The generic percpu_op() should be local to the cpu, so it should > expand to... > > do { get_cpu_var(var) OP (val); put_cpu_var(var) } while (0) > > as the original x86_OP_percpu() did. Right? > > Thanks.
hm, that removes much of its appeal - a preempt off+on sequence is quite bloaty. Most percpu usage sites are already within critical sections.
I think they are more analogous to per_cpu(var, cpu), which does not disable preemption either. There's no 'get/put' in them, which signals that they dont auto-disable preemption.
Ingo
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