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

Ingo Molnar
2012-05-21T14:08:30Z

* Vlad Zolotarov <vlad@scalemp.com> wrote:

> 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...

Well, a quick tally of percpu variables on a 'make defconfig'
kernel would tell us one way or another?

Here there's almost 200 percpu variables active in the 64-bit
x86 defconfig, and a quick random sample suggests that most are
read-mostly.

I have no fundamental prefer to either approach, but the
direction taken should be justified explicitly, with numbers,
arguments, etc. - also a short blurb somewhere in the headers
that explains when they should be used, so that others can be
aware of vSMP's special needs here.

Thanks,

Ingo