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

Vlad Zolotarov
2012-05-21T14:57:01Z

On Monday, May 21, 2012 16:08:22 Ingo Molnar wrote:
> * 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.

There must be some misunderstanding - this patch is not a vSMP Foundation
specific as it defines read-mostly variables as __read_mostly. The motivation
for it is just the same as in a non-vSMP Foundation case. It's true that the
performance gain this patch introduces in the vSMP Foundation is likely to be
more significant than in a native Linux, however even for a native Linux it
would still be a better code as __read_mostly is not a vSMP Foundation
specific paradigm and, again, the variables modified are a clear read-mostly
case.

So, the explanation u request above would be just the same as if I would
explain when in general __read_mostly should be used.

I grep'ed the Documentation and haven't found any readme file with the
explicit instructions when __read_mostly qualifier should be used and u r
right we'd better write one.

I can create an initial version of such a doc but I think it would better come
as a separate patch.

May we advance this way?

Pls., comment.

>
> Thanks,
>
> Ingo
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