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Robert Love wrote: > > ... > > Question: if (from below) you are going to use atomic operations, why > make it per-CPU at all? Just have one counter and atomic_inc and > atomic_read it. You won't need a spin lock. Oh that works fine. But then it's a global counter, so each time a CPU marks a page dirty, the counter needs to be pulled out of another CPU's cache. Which is not a thing I *need* to do. As I said, it's a micro-issue. But it's a requirement which may pop up elsewhere. > This would be atomic and thus preempt-safe on any sane arch I know, as > long as we are dealing with a normal type int. Admittedly, however, we > can't be sure what the compiler would do. > > Thinking about it, you are probably going to be doing this: > > ++counter[smp_processor_id()]; > > and that is not preempt-safe since the whole operation certainly is not > atomic. The current CPU could change between calculating it and > referencing the array. yup. It'd probably work - the compiler should calculate the address and do a non-buslocked but IRQ-atomic increment on it. But no way can we rely on that happening. > But, that wouldn't matter as long as you only > cared about the sum of the counters. If the compiler produced code equivalent to counter[smp_processor_id()] = counter[smp_processor_id()] + 1; then the counter would get trashed - a context switch could cause CPUB to write CPUA's counter (+1) onto CPUB's counter. It's quite possibly illegal for the compiler to evaluate the array subscript twice in this manner. Dunno. If the compiler produced code equivalent to: const int cpu = smp_processor_id(); counter[cpu] = counter[cpu] + 1; (which is much more likely) then a context switch would result in CPUB writing CPUA's updated counter onto CPUA's counter. Which will work a lot more often, until CPUA happens to be updating its counter at the same time. > ... > > 2: In <linux/atomic.h>, > > > > #ifndef ARCH_HAS_ATOMIC_INQ_THINGIES > > #define atomic_inc_irq atomic_inc > > ... > > #endif > > I can think up a few more uses of the irq/memory-safe atomic ops, so I > bet this isn't that bad of an idea. But no point doing it without a > corresponding use. Sure. - - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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