lkml.org 
[lkml]   [2007]   [May]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/2] scalable rw_mutex
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/

\
 
 \ /
  Last update: 2007-05-12 20:09    [W:0.153 / U:0.240 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site