Re: [Lse-tech] Re: [Patch 2/7] Add sysctl for schedstats

[Nick Piggin]

Chandra Seetharaman wrote:

> On Mon, 2006-02-27 at 20:17 +1100, Nick Piggin wrote:
>
> > > #ifdef CONFIG_SCHEDSTATS
> > > +
> > > +int schedstats_sysctl = 0;		/* schedstats turned off by default */
> > Should be read mostly.
> >
> >
> > > +static DEFINE_PER_CPU(int, schedstats) = 0;
> > > +
> > When the above is in the read mostly section, you won't need this at all.
> >
> > You don't intend to switch the sysctl with great frequency, do you?
>
> No, it is not expected to switch often.
>
> We originally coded it as __read_mostly, but thought the variable
> bouncing between CPUs would be costly. Is it cheaper with
> __read_mostly ? or it doesn't matter ?

Well it will only "bounce" when the cacheline it is in is written to by
a different CPU. Considering this happens with your per-cpu implementation
_anyway_, they don't buy you anything much.

Putting it in __read_mostly means that you won't happen to share a cacheline
with a variable that is being written to frequently.

Nick

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