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Date: Wed, 28 Jan 2009 21:08:49 +1030 From: Rusty Russell <rusty@...tcorp.com.au> To: Tejun Heo <tj@...nel.org> Cc: Ingo Molnar <mingo@...e.hu>, Herbert Xu <herbert@...dor.apana.org.au>, akpm@...ux-foundation.org, hpa@...or.com, brgerst@...il.com, ebiederm@...ssion.com, cl@...ux-foundation.org, travis@....com, linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com, "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org, Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> Subject: Re: [PATCH] percpu: add optimized generic percpu accessors On Tuesday 27 January 2009 12:54:27 Tejun Heo wrote: > Hello, Rusty. Hi Tejun! > There actually were quite some places where atomic add ops would be > useful, especially the places where statistics are collected. For > logical bitops, I don't think we'll have too many of them. If the stats are only manipulated in one context, than an atomic requirement is overkill (and expensive on non-x86). > > If they are worth doing generically, should the ops be atomic? To > > extrapolate from x86 usages again, it seems to be happy with > > non-atomic (tho of course it is atomic on x86). > > If atomic rw/add/sub are implementible on most archs (and judging from > local_t, I suppose it is), I think it should. So that it can replace > local_t and we won't need something else again in the future. This is more like Christoph's CPU_OPS: they were special operators on normal per-cpu vars/ptrs. Generic version was irqsave+op+irqrestore. I actually like this idea, but Mathieu insists that the ops be NMI-safe, for ftrace. Hence local_t needing to be atomic_t for generic code. AFAICT we'll need a hybrid: HAVE_NMISAFE_CPUOPS, and if not, use atomic_t in ftrace (which isn't NMI safe on parisc or sparc/32 anyway, but I don't think we care). Other than the shouting, I liked Christoph's system: - CPU_INC = always safe (eg. local_irq_save/per_cpu(i)++/local_irq_restore) - _CPU_INC = not safe against interrupts (eg. get_cpu/per_cpu(i)++/put_cpu) - __CPU_INC = not safe against anything (eg. per_cpu(i)++) I prefer the name 'local' to the name 'cpu', but I'm not hugely fussed. > >> Another question to ask is whether to keep using separate > >> interfaces for static and dynamic percpu variables or migrate to > >> something which can take both. > > > > Well, IA64 can do stuff with static percpus that it can't do with > > dynamic (assuming we get expanding dynamic percpu areas > > later). That's because they use TLB tricks for a static 64k per-cpu > > area, but this doesn't scale. That might not be vital: abandoning > > that trick will mean they can't optimise read_percpu/read_percpu_var > > etc as much. > > Isn't something like the following possible? > > #define pcpu_read(ptr) \ > ({ \ > if (__builtin_constant_p(ptr) && \ > ptr >= PCPU_STATIC_START && ptr < PCPU_STATIC_END) \ > do 64k TLB trick for static pcpu; \ > else \ > do generic stuff; \ > }) No, that will be "do generic stuff", since it's a link-time constant. I don't know that this is a huge worry, to be honest. We can leave the __ia64_per_cpu_var for their arch-specific code (I feel the same way about x86 to be honest). > > Tejun, any chance of you updating the tj-percpu tree? My current > > patches are against Linus's tree, and rebasing them on yours > > involves some icky merging. > > If Ingo is okay with it, I'm fine with it too. Unless Ingo objects, > I'll do it tomorrow-ish (still big holiday here). Ah, I did not realize that you celebrated Australia day :) Cheers! Rusty. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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