On 12 May 2007 20:55:28 +0200 Andi Kleen <[email protected]> wrote:
> Andrew Morton <[email protected]> writes:
>
> > On Fri, 11 May 2007 10:07:17 -0700 (PDT)
> > Christoph Lameter <[email protected]> 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).
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