Re: [PATCH v3 0/2] Move x86_cpu_to_apicid to the __read_mostly section

Vlad Zolotarov
2012-05-21T13:54:31Z

On Monday, May 21, 2012 02:32:46 PM Ingo Molnar wrote:
> * Shai Fultheim (Shai@ScaleMP.com) <Shai@ScaleMP.com> wrote:
> > Ingo,
> >
> > The reason for this, as you pointed out, is the 'cache line'
> > size (4096 bytes). We see significant false sharing is we do
> > not move this next to each other.
>
> Which write-often variable caused the many cache flushes/fills?
> cpu_to_apicid is read mostly.
>
> I.e. it might make more sense to identify the frequenty
> *modified* percpu variables, and move them to a separate
> section. I *think* most percpu variables are read mostly, so it
> would be more maintainable in the long run to figure out the
> frequently modified ones, not the frequently not modified ones.

I tend to disagree about the general claim that most per-CPU variables are
read-mostly: consider the per-CPU data structures used in lock-less algorithms
like softnet_data used in a NAPI. I'm not sure what is a more common - read-
only or not-read-only per-cpu data, but surely there are both...

In this specific patch we deal with something that is initialized once in the
init time and then used as if it's a constant thus representing a clear
"__read_mostly" case.

Having said all that I think that the proposed solution, the one using
__read_mostly infrastructure, is just ok both in a long run. I also doubt that
we are currently facing a need to define an additional "frequently modified"
section.

Pls., comment.

thanks,
vlad

>
> Thanks,
>
> Ingo