Messages in this thread | | | Date | Sat, 12 May 2007 11:06:24 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 1/2] scalable rw_mutex |
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On 12 May 2007 20:55:28 +0200 Andi Kleen <andi@firstfloor.org> wrote:
> Andrew Morton <akpm@linux-foundation.org> writes: > > > On Fri, 11 May 2007 10:07:17 -0700 (PDT) > > Christoph Lameter <clameter@sgi.com> wrote: > > > > > On Fri, 11 May 2007, Andrew Morton wrote: > > > > > > > yipes. percpu_counter_sum() is expensive. > > > > > > Capable of triggering NMI watchdog on 4096+ processors? > > > > Well. That would be a millisecond per cpu which sounds improbable. And > > we'd need to be calling it under local_irq_save() which we presently don't. > > And nobody has reported any problems against the existing callsites. > > > > But it's no speed demon, that's for sure. > > There is one possible optimization for this I did some time ago. You don't really > need to sum all over the possible map, but only all CPUs that were ever > online. But this only helps on systems where the possible map is bigger > than online map in the common case. But that shouldn't be the case anymore on x86 > -- it just used to be. If it's true on some other architectures it might > be still worth it. >
hm, yeah.
We could put a cpumask in percpu_counter, initialise it to cpu_possible_map. Then, those callsites which have hotplug notifiers can call into new percpu_counter functions which clear and set bits in that cpumask and which drain percpu_counter.counts[cpu] into percpu_counter.count.
And percpu_counter_sum() gets taught to do for_each_cpu_mask(fbc->cpumask). - 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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